[clang][test] Make headers unique to avoid linking issues

Make the empty headers used by cl-pch-showincludes.cpp unique so that
filesystems that link these files together by contents will not see
different behaviour in this test, which is not testing linked files
specifically.

This was uncovered by 5ea78c41 which made us stop mutating the name
of the presumed loc for the file in ContentCache, but that just surfaced
an underlying issue that the filename of multiple includes of linked
files are not separately tracked.

Differential Revision: https://reviews.llvm.org/D135373