Some vendor tools convert PDB file names given on the command line to
lower-case before creating the file.  When CMake places a mixed-case PDB
file name into the build system, the file does not exist the first time
and it is written with mixed case.  After the first build though the
native tool has created a lower-case version of the file.  If CMake does
CollapseFullPath again, the file exists so the actual-case lookup gets
the lower-case name.  This causes the build files to change so the
project rebuilds.

The solution is to avoid calling CollapseFullPath for files generated by
the build.  In the case of PDB files we already construct them from
paths that have been collapsed, so we can just skip the call altogether.
See issue #9350.