Instead of deprecating the "gch probe" as in f726da1193baf51e0a66453cc32dcffb8a9121d4, this makes clang ignore files which are not clang pch files (See discussion on PR #67084). This fixes the issues mentioned in the former patch, with GCC-generated .gch files getting in the way when using clang, while maintaining the probing behavior for builds which rely on that.