[clang] [clang][Tooling] Insert global module fragment during header insertion (PR #173724)

[Sirui Mu via cfe-commits]

Lancern wrote:

> Maybe we can do something similar in https://github.com/llvm/llvm-project/issues/172966
>
> e.g, we should ask the tools to insert import std; instead of #include.
>
> We can file an issue for it as the first step.

The first step could be adding `import <header>` support to `HeaderIncludes`. Currently `HeaderIncludes` only support `#include` and `#import`.

https://github.com/llvm/llvm-project/pull/173724