[AIX] Add git revision to .file string (#88164)

If `LLVM_APPEND_VC_REV` is on, add the git revision to the `.file`
string. The revision can be set with `LLVM_FORCE_VC_REVISION`.

Before:
`.file	"git_revision.cpp",,"LLVM version 19.0.0git"`

After:
`.file	"git_revision.cpp",,"LLVM version 19.0.0git (LLVM_REVISION)"`