[clang][Driver] Don't ignore -gmodules .gch files (#77711)

A previous commit (82f75ed5) made clang ignore .gch files that were not
Clang AST files. This broke `-gmodules`, which embeds the Clang AST into
an object file containing debug info.

This changes the probing to detect any file format recognized by
`llvm::identify_magic()` as potentially containing a Clang AST.

Previous PR: https://github.com/llvm/llvm-project/pull/69204