Allow retrieving source files relative to the compilation directory.

Allow retrieving source files relative to the compilation directory.

Summary:
Dwarf stores source-file names the three parts:
<compilation_directory><include_directory><filename>

Prior to this change, the code only allowed retrieving either all
three as the absolute path, or just the filename.  But many
compile-command lines--especially those in hermetic build systems
don't specify an absolute path, nor just the filename, but rather the
path relative to the compilation directory. This features allows
retrieving them in that style.

Add tests for path printing styles.

Modify createBasicPrologue to handle include directories.

Subscribers: aprantl, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73383