[Dwarf] Optimize getOrCreateSourceID() for repeated calls on same file (NFCI)

DwarfCompileUnit::getOrCreateSourceID() is often called many times
in sequence with the same DIFile. This is currently very expensive,
because it involves creating a string from directory and file name
and looking it up in a string map. This patch remembers the last
DIFile and its ID and directly returns that.

This gives a geomean -1.3% compile-time improvement on CTMark O0-g.

Differential Revision: https://reviews.llvm.org/D118041