This commit enables 'llvm-dwarfdump --veriy' to verify the DWARF in
foreign type units when using split DWARF for the .debug_names section.

This patch adds support for the new foreign type unit support in
.debug_names. Features include:
- don't manually index foreign TUs if we have info for them
- only use the type unit entries that match the .dwo files when we have
a .dwp file
- fix type unit lookups for .dwo files
- fix crashers that happen due to PeekDIEName() using wrong offsets where an entry had DW_IDX_comp_unit and DW_IDX_type_unit entries and when we had no type unit support, it would cause us to think it was a normal DIE in .debug_info from the main executable.

---------

Co-authored-by: paperchalice <liujunchang97@outlook.com>

The parameter of `findDebugNamesOffsets` has been renamed to
`EndOfHeaderOffset` in #88064 to make it clear it is a section offset
instead of an offset relative to the current name index. Rename the call
site variable as well.

Address some post-review comments in #82153 and move the function inside
llvm::dwarf, used by certain free functions.

Pull Request: https://github.com/llvm/llvm-project/pull/88064

Refactor the code that calculates the offsets for the various pieces of
the DWARF .debug_names index section, to make it easier to share the
code with other tools, such as LLD.

This stemps from conversatin in:
https://github.com/llvm/llvm-project/pull/77457#discussion_r1457889792.
Right now Abbrev code for abbrev is combination of DIE TAG and other
attributes.
In the future it will be changed to be an index. Since DenseSet does not
preserve an order, added a sort based on abbrev code. Once change to
index is
made, it will print out abbrevs in the order they are stored.

TableEntry names are pointers into the string table section, and
accessing their
length requires a search for `\0`. However, 99% of the time we only need
to
compare the name against some other other, and such a comparison will
fail as
early as the first character.

This commit adds a method to the interface of TableEntry so that such a
comparison can be done without extracting the full name. It saves 10% in
the
time (1250ms -> 1100 ms) to evaluate the following expression.

```
lldb \
  --batch \
  -o "b CodeGenFunction::GenerateCode" \
  -o run \
  -o "expr Fn" \
  -- \
  clang++ -c -g test.cpp -o /dev/null &> output
```

The current implementation of DebugNames is _only_ using hashes to
compute the bucket number. Once inside the bucket, it reverts back to
string comparisons, even though not all hashes inside a bucket are
identical.

This commit changes the behavior so that we check the hash before
comparing strings. Such check is so important that it speeds up a simple
benchmark by 20%. In other words, the following expression evaluation
time goes from 1100ms to 850ms.

```
bin/lldb \
		--batch \
		-o "b CodeGenFunction::GenerateCode" \
		-o run \
		-o "expr Fn" \
		-- \
		clang++ -c -g test.cpp -o /dev/null &> output
```

(Note, these numbers are considering the usage of IDX_parent)

This commit introduces a helper function to DWARFAcceleratorTable::Entry
which follows DW_IDX_Parent attributes to returns the corresponding
parent Entry in the table.

It is tested by enhancing dwarfdump so that it now prints:

1. When data is corrupt.
2. When parent information is present, but the parent is not indexed.
3. The parent entry offset, when the parent is present and indexed. This
is printed in terms a real entry offset (the same that gets printed at
the start of each entry: "Entry @ 0x..."), instead of the encoded number
in the table (which is an offset from the start off the Entry list).
This makes it easy to visually inspect the dwarfdump and check what the
parent is.

This is a follow up patch after .debug_names can now emit local type
unit entries when we compile with type units + DWARF5 + .debug_names.
The pull request that added this functionality was:

https://github.com/llvm/llvm-project/pull/70515

This patch makes sure that the DebugNamesDWARFIndex in LLDB will not
manually need to parse type units if they have a valid index. It also
fixes the index to be able to correctly extract name entries that
reference type unit DIEs. Added a test to verify things work as
expected.