DebugInfo: Avoid creating DILexicalScopeFiles when the filename in the current scope has not changed.

This looks like the right way for this check to work, but there is
another semi-obvious bug, I would think: why is CurLoc not zero'd out
between functions? The possibility for it to bleed between them seems
problematic. (& indeed I caused tests to fail when I fixed this a
different way, by setting CurLoc to SourceLocation() and the end of
EmitFunctionEnd... )

The changes to debug-info-blocks.m are due to a mismatch between the
source manager's file naming and CGDebugInfo's default handling when no
-main-file-name is specified. This actually reveals somewhat of a bug in
the debug info when using source files from standard in, too. See the
comment in CGDebugInfo::CreateCompileUnit for more details.