Today the front-ends (llvm-gcc and clang) generate multiple llvm.dbg.compile_units to identify source file for various debug entities. Each llvm.dbg.compile_unit matches one file on the disk. However, the backend only supports one DW_TAG_compile_unit per .o file. The backend selects first compile_unit from the vector to construct DW_TAG_compile_unit entry, which is not correct in all cases.

First step to resolve this is, record file name and directory directly in debug info for various debug entities. 

llvm-svn: 61164