Teach the IR parser and writer to support metadata on ifuncs, and update
documentation.

In PR #153049, we have a use case of attaching the `!associated`
metadata to an ifunc.
Since an ifunc is similar to a function declaration, it seems natural to
allow metadata on ifuncs.
Currently, the metadata API allows adding Metadata to
llvm::GlobalObject, so the in-memory IR allows for metadata on ifuncs,
but the IR reader/writer is not aware of that.

---------

Co-authored-by: Wael Yehia <wyehia@ca.ibm.com>

Reapply "IR: Remove uselist for constantdata (#137313)"

This reverts commit 5936c02c8b9c6d1476f7830517781ce8b6e26e75.

Fix checking uselists of constants in assume bundle queries

Possibly breaks the build: https://lab.llvm.org/buildbot/#/builders/24/builds/8119

This reverts commit 87f312aad6ede636cd2de5d18f3058bf2caf5651.

This is a resurrected version of the patch attached to this RFC:

https://discourse.llvm.org/t/rfc-constantdata-should-not-have-use-lists/42606

In this adaptation, there are a few differences. In the original patch, the Use's
use list was replaced with an unsigned* to the reference count in the value. This
version leaves them as null and leaves the ref counting only in Value.

Remove use-lists from instances of ConstantData (which are shared
across modules and have no operands).

To continue supporting most of the use-list API, store a ref-count in
place of the use-list; this is for API like Value::use_empty and
Value::hasNUses.  Operations that actually need the use-list -- like
Value::use_begin -- will assert.

This change has three benefits:

 1. The compiler output cannot in any way depend on the use-list order
    of instances of ConstantData.

 2. There's no use-list traffic when adding and removing simple
    constants from operand lists (although there is ref-count traffic;
    YMMV).

 3. It's cheaper to serialize use-lists (since we're no longer
    serializing the use-list order of things like i32 0).

The downside is that you can't look at all the users of ConstantData,
but traversals of users of i32 0 are already ill-advised.

Possible follow-ups:
  - Track if an instance of a ConstantVector/ConstantArray/etc. is known
    to have all ConstantData arguments, and drop the use-lists to
    ref-counts in those cases.  Callers need to check Value::hasUseList
    before iterating through the use-list.
  - Remove even the ref-counts.  I'm not sure they have any benefit
    besides minimizing the scope of this commit, and maintaining the
    counts is not free.

Fixes #58629

Co-authored-by: Duncan P. N. Exon Smith <dexonsmith@apple.com>

This patch renames DPLabel to DbgLabelRecord, in accordance with the
ongoing DbgRecord rename. This rename was fairly trivial, since DPLabel
isn't as widely used as DPValue and has no real conflicts in either its
full or abbreviated name. As usual, the entire replacement was done
automatically, with `s/DPLabel/DbgLabelRecord/` and `s/DPL/DLR/`.

This is the major rename patch that prior patches have built towards.
The DPValue class is being renamed to DbgVariableRecord, which reflects
the updated terminology for the "final" implementation of the RemoveDI
feature. This is a pure string substitution + clang-format patch. The
only manual component of this patch was determining where to perform
these string substitutions: `DPValue` and `DPV` are almost exclusively
used for DbgRecords, *except* for:

- llvm/lib/target, where 'DP' is used to mean double-precision, and so
appears as part of .td files and in variable names. NB: There is a
single existing use of `DPValue` here that refers to debug info, which
I've manually updated.
- llvm/tools/gold, where 'LDPV' is used as a prefix for symbol
visibility enums.

Outside of these places, I've applied several basic string
substitutions, with the intent that they only affect DbgRecord-related
identifiers; I've checked them as I went through to verify this, with
reasonable confidence that there are no unintended changes that slipped
through the cracks. The substitutions applied are all case-sensitive,
and are applied in the order shown:

```
  DPValue -> DbgVariableRecord
  DPVal -> DbgVarRec
  DPV -> DVR
```

Following the previous rename patches, it should be the case that there
are no instances of any of these strings that are meant to refer to the
general case of DbgRecords, or anything other than the DPValue class.
The idea behind this patch is therefore that pure string substitution is
correct in all cases as long as these assumptions hold.

Reaplying after revert in #85382 (861ebe6446296c96578807363aa292c69d827773).
Fixed intermittent test failure by avoiding piping output in some RUN lines.

If --write-experimental-debuginfo-iterators-to-bitcode is true (default false)
and --expermental-debuginfo-iterators is also true then the new debug info
format (non-instruction records) is written to bitcode directly.

Added the following records:

    FUNC_CODE_DEBUG_RECORD_LABEL
    FUNC_CODE_DEBUG_RECORD_VALUE
    FUNC_CODE_DEBUG_RECORD_DECLARE
    FUNC_CODE_DEBUG_RECORD_ASSIGN
    FUNC_CODE_DEBUG_RECORD_VALUE_SIMPLE

The last one has an abbrev in FUNCTION_BLOCK BLOCK_INFO. Incidentally, this uses
the last value available without widening the code-length for FUNCTION_BLOCK
from 4 to 5 bits.

Records are formatted as follows:

    All DbgRecord start with:
      1. DILocation

      FUNC_CODE_DEBUG_RECORD_LABEL
        2. DILabel

      DPValues then share common fields:
        2. DILocalVariable
        3. DIExpression

        FUNC_CODE_DEBUG_RECORD_VALUE
          4. Location Metadata

        FUNC_CODE_DEBUG_RECORD_DECLARE
          4. Location Metadata

        FUNC_CODE_DEBUG_RECORD_VALUE_SIMPLE
	  4. Location Value (single)

        FUNC_CODE_DEBUG_RECORD_ASSIGN
	  4. Location Metadata
	  5. DIAssignID
	  6. DIExpression (address)
	  7. Location Metadata (address)

Encoding the DILocation metadata reference directly appeared to yield smaller
bitcode files than encoding the operands seperately (as is done with instruction
DILocations).

FUNC_CODE_DEBUG_RECORD_VALUE_SIMPLE is by far the most common DbgRecord record
in optimized code (order of 5x-10x over other kinds). Unoptimized code should
only contain FUNC_CODE_DEBUG_RECORD_DECLARE.