Try to give a more complete description of what we call a "fragment",
and how debug records interact when fragments are involved.

This patch introduces a new option `-preserve-merged-debug-info` to
preserve an arbitrary but deterministic version of debug information
when DILocations are merged. This is intended to be used in production
environments from which sample based profiles are derived such as
AutoFDO and MemProf.

With this patch we have see a 0.2% improvement on an internal workload
at Google when generating AutoFDO profiles. It also significantly
improves the ability for MemProf by preserving debug info for merged
call instructions used in the contextual profile.

---------

Co-authored-by: Krzysztof Pszeniczny <kpszeniczny@google.com>

As far as I understand, the first operand of dbg_declare should be a
pointer (inside a metadata wrapper). However, using a non-pointer is
currently not rejected, and we have some tests that use non-pointer
types. As far as I can tell, these tests either meant to use dbg_value
or are just incorrect hand-crafted tests.

Ran into this while trying to `fix` #134008.

The metadata was off-by-one in a couple of places, making the
explanatory text incorrect.

As we approach the state where support for debug intrinsics is dropping and
we print and use debug records by default, the documentation should be updated
to refer to debug records as the primary debug info representation, with
debug intrinsics being relegated to an optional alternative.

This patch performs a few updates:
- Replace references to intrinsics with references to records across all
the documentation.
- Replace intrinsics with records in code examples.
- Move debug records prior to debug intrinsics in the
SourceLevelDebugging document, and change text to refer to them as the
primary representation.
- Add release notes describing the change.

This patch adds minimal documentation for the IR representation of the
RemoveDIs model of debug info. This patch assumes
that the default for all cases is still debug intrinsic functions, and
so does not update all existing text to refer to debug records, but only
adds a section in the LangRef and SourceLevelDebugging documents to
explain the new format.

An SSA register `value` was being defined twice.
An "external" link was using the "internal" link syntax.

Commit a93c4239719382e5b17335f3452e9095937ed6b7 broke docs buildbot:
https://lab.llvm.org/buildbot/#/builders/30/builds/34525

Reviewed By: scott.linder, jryans

Differential Revision: https://reviews.llvm.org/D140989