[Modules] Process include files changes (#90319)

There were two diffs that introduced some options useful when you build
modules externally and cannot rely on file modification time as the key
for detecting input file changes:
- [D67249](https://reviews.llvm.org/D67249) introduced the
`-fmodules-validate-input-files-content` option, which allows the use of
file content hash in addition to the modification time.
- [D141632](https://reviews.llvm.org/D141632) propagated the use of
`-fno-pch-timestamps` with Clang modules.

There is a problem when the size of the input file (header) is not
modified but the content is. In this case, Clang cannot detect the file
change when the `-fno-pch-timestamps` option is used. The
`-fmodules-validate-input-files-content` option should help, but there
is an issue with its application: it's not applied when the modification
time is stored as zero that is the case for `-fno-pch-timestamps`.

The issue can be fixed using the same trick that was applied during the
processing of `ForceCheckCXX20ModulesInputFiles`:
```
  // When ForceCheckCXX20ModulesInputFiles and ValidateASTInputFilesContent
  // enabled, it is better to check the contents of the inputs. Since we can't
  // get correct modified time information for inputs from overriden inputs.
  if (HSOpts.ForceCheckCXX20ModulesInputFiles && ValidateASTInputFilesContent &&
      F.StandardCXXModule && FileChange.Kind == Change::None)
    FileChange = HasInputContentChanged(FileChange);
```
The patch suggests the solution similar to the presented above and
includes a LIT test to verify it.