[clang] [clang] Implement function pointer signing and authenticated function calls (PR #93906)

[Hana Dusíková via cfe-commits]

hanickadot wrote:

When this change is building at Compiler Explorer I noticed it spams build output with warnings:
```
In file included from /root/llvm-project/clang/include/clang/AST/Type.h:31,
                 from /root/llvm-project/clang/include/clang/AST/DeclarationName.h:16,
                 from /root/llvm-project/clang/include/clang/AST/DeclBase.h:19,
                 from /root/llvm-project/clang/include/clang/AST/Decl.h:20,
                 from /root/llvm-project/clang/tools/libclang/CXExtractAPI.cpp:20:
/root/llvm-project/clang/include/clang/Basic/PointerAuthOptions.h:61:8: warning: 'clang::PointerAuthSchema::TheKind' is too small to hold all values of 'enum class clang::PointerAuthSchema::Kind'
   61 |   Kind TheKind : 2;
      |        ^~~~~~~
/root/llvm-project/clang/include/clang/Basic/PointerAuthOptions.h:65:29: warning: 'clang::PointerAuthSchema::SelectedAuthenticationMode' is too small to hold all values of 'enum class clang::PointerAuthenticationMode'
   65 |   PointerAuthenticationMode SelectedAuthenticationMode : 2;
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~
/root/llvm-project/clang/include/clang/Basic/PointerAuthOptions.h:66:18: warning: 'clang::PointerAuthSchema::DiscriminationKind' is too small to hold all values of 'enum class clang::PointerAuthSchema::Discrimination'
   66 |   Discrimination DiscriminationKind : 2;
      |                  ^~~~~~~~~~~~~~~~~~
      ```
compiler-explorar pipe line builds it with GCC 9.2

https://github.com/llvm/llvm-project/pull/93906