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

[Jake Egan via llvm-commits]

jakeegan wrote:

Hi, on AIX we are seeing 
```
error: input is not a PCH file: '/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/tools/clang/test/PCH/Output/gch-probe.c.tmp.h.gch'
fatal error: file '/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/tools/clang/test/PCH/Output/gch-probe.c.tmp.h.gch' is not a valid precompiled PCH file: file too small to contain AST file magic
```
https://lab.llvm.org/buildbot/#/builders/214/builds/11018/steps/6/logs/FAIL__Clang__gch-probe_c

Could you take a look please?

https://github.com/llvm/llvm-project/pull/77711