TARGET_COMPILE_PDB_FILE: Add genex to get compiler PDB file
Discussion in #16932 (comment 273512) identified that we do not have a generator expression to get the location of the compiler-generated program database file associated with a target. We already have TARGET_PDB_FILE
for the linker-generated PDB file. A new TARGET_COMPILE_PDB_FILE
could be added for the compiler-generated PDB file.