Do not call CollapseFullPath for PDB file names
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.