[clang] [FMV] Emit the resolver along with the default version definition. (PR #84405)

[Daniel Kiss via cfe-commits]

DanielKristofKiss wrote:

I'd like to support FMV in existing codebases as lean as possible, so the default version attribute would be optional to write as not all version/toolchain will support it. smallest possible codebase change to introduce multi versioning:

```c
 int foo(void);

+ #ifdef __HAVE_FUNCTION_MULTI_VERSIONING
+ int __attribute__((target_version("feature"))) foo(void);
+ #endif
```

https://github.com/llvm/llvm-project/pull/84405