The gcov version is set to 11.1 (compatible with gcov 9) even if
`-Xclang -coverage-version=` specified version is less than 11.1.

Therefore, we can drop producer support for version < 11.1.

`OptimizationLevel` and `OptimizeSize` can affect the generated AST. They indirectly affect the `Optimize` and `OptimizeSize` frontend options, which in turn set predefined macro definitions.

This fixes rdar://121228252.

CodeGen options do not affect the AST, so they usually can be ignored.
The only exception to the rule is when a PCM is created with
`-gmodules`.
In that case the Clang module format is switched to object file
container and contains also serialized debug information that can be
affected by debug options. There the following approach was choosen:

1.) Split out all the debug options into a separate `DebugOptions.def`
    file. The file is included by `CodeGenOptions.def`, so the change is
    transparent to all existing users of `CodeGenOptions.def`.
2.) Reset all CodeGen options, but excluding affecting debug options.
3.) Conditionally reset debug options that can affect the PCM.

This fixes rdar://113135909.

This reverts commit fef1854318bd797c1f8a141d4b45b113b04860d1.

CodeGen options do not affect the AST, so they usually can be ignored.
The only exception to the rule is when a PCM is created with
`-gmodules`.
In that case the Clang module format is switched to object file
container and contains also serialized debug information that can be
affected by debug options. There the following approach was choosen:

1.) Split out all the debug options into a separate `DebugOptions.def`
    file. The file is included by `CodeGenOptions.def`, so the change is
    transparent to all existing users of `CodeGenOptions.def`.
2.) Reset all CodeGen options, but excluding affecting debug options.
3.) Conditionally reset debug options that can affect the PCM.

This fixes rdar://113135909.

It's available both in CodeGenOptions and in LangOptions, and LangOptions
implementation is slightly better as it uses a StringRef instead of a char
pointer, so use it.

Differential Revision: https://reviews.llvm.org/D98175

This reverts commit c4a2a1307484cffe94a291c42572775411bac8d8.

This commit was causing a test failure: http://lab.llvm.org:8011/builders/llvm-clang-win-x-armv7l/builds/1068

Once the new option parsing system is committed, this will allow to generate a
check to ensure that correct command line generation happens

Differential Revision: https://reviews.llvm.org/D86290

gcov 4.8 (r189778) moved the exit block from the last to the second.
The .gcda format is compatible with 4.7 but

* decoding libgcov 4.7 produced .gcda with gcov [4.7,8) can mistake the
  exit block, emit bogus `%s:'%s' has arcs from exit block\n` warnings,
  and print wrong `" returned %s` for branch statistics (-b).
* decoding libgcov 4.8 produced .gcda with gcov 4.7 has similar issues.

Also, rename "return block" to "exit block" because the latter is the
appropriate term.

Defaulting to -Xclang -coverage-version='407*' makes .gcno/.gcda
compatible with gcov [4.7,8)

In addition, delete clang::CodeGenOptionsBase::CoverageExtraChecksum and GCOVOptions::UseCfgChecksum.
We can infer the information from the version.

With this change, .gcda files produced by `clang --coverage a.o` linked executable can be read by gcov 4.7~7.
We don't need other -Xclang -coverage* options.
There may be a mismatching version warning, though.

(Note, GCC r173147 "split checksum into cfg checksum and line checksum"
 made gcov 4.7 incompatible with previous versions.)