[C++20] [Modules] [Serialization] Check input file contents with option...

[C++20] [Modules] [Serialization] Check input file contents with option ForceCheckCXX20ModulesInputFiles enabled

With overriden input files, e,g,. the compiler get the file from an
in-memroy buffer, the compiler can't get correct modified time information
to indicate whehter the input files are changed or not. Then, the
semantics of ForceCheckCXX20ModulesInputFiles are broken.

In this patch, if both ForceCheckCXX20ModulesInputFiles and
ValidateASTInputFilesContent and enabled, the compiler will still check the
hash value of the contents even if their modification time is the same.