[cxxmodules][pch] Disable validation of transent files.

In clang a transient file is a header file which is stored in the pch/pcm file.
If this file does not exist, clang creates a virtual file of the unzipped stored
file. However, if the file exists it compares the size of the file on disk and
the size of the transent file. If the two sizes are different it does not
complain (as validation is off) but it triggers an assert in when constructing
and outdated clang::InputFile. The implementation disallows a overridden or
transient file to be out of date.

This patch extends the DisableValidation abilities to span not only for timestamps
but we avoid checking the file sizes if the file is on the disk. And, instead we
just create a virtual file of the file that we already have.

This patch should fix a relocation issue for cmssw and some of the reported LCG
issues.