[clang] Fix ASTWriter crash after merging named enums (#114240)

Clang already removes parsed enumerators when merging typedefs to
anonymous enums. This is why the following example decl used to be
handled correctly while merging, and ASTWriter behaves as expected:

```c
typedef enum { Val } AnonEnum;
```
However, the mentioned mechanism didn't handle named enums. This leads
to stale declarations in `IdResolver`, causing an assertion violation in
ASTWriter ``Assertion `DeclIDs.contains(D) && "Declaration not
emitted!"' failed`` when a module is being serialized with the following
example merged enums:

```c
typedef enum Enum1 { Val_A } Enum1;
enum Enum2 { Val_B };
```

The PR applies the same mechanism in the named enums case.

Additionally, I dropped the call to
`getLexicalDeclContext()->removeDecl` as it was causing a wrong
odr-violation diagnostic with anonymous enums.

Might be easier to to review commit by commit. Any feedback is
appreciated.

### Context

This fixes frontend crashes that were encountered when certain
Objective-C modules are included on Xcode 16. For example, by running
`CC=/path/to/clang-19 xcodebuild clean build` on a project that contains
the following Objective-C file:

```c
#include <os/atomic.h>

int main() {
  return memory_order_relaxed;
}
```
This crashes the parser in release, when ASTReader tries to load the
enumerator declaration.
6 files changed
tree: 94e0b94fdcd740ea7932cd13d99f2ed44404d647
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-tidy
  30. .git-blame-ignore-revs
  31. .gitattributes
  32. .gitignore
  33. .mailmap
  34. CODE_OF_CONDUCT.md
  35. CONTRIBUTING.md
  36. LICENSE.TXT
  37. pyproject.toml
  38. README.md
  39. SECURITY.md
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