Reuse code from CGDebugInfo::getOrCreateFile() when creating the file

for the DICompileUnit.

This addresses post-commit feedback for D55085. Without this patch, a
main source file with an absolute paths may appear in different
DIFiles, once with the absolute path and once with the common prefix
between the absolute path and the current working directory.

Differential Revision: https://reviews.llvm.org/D55519

llvm-svn: 348865