COFF: add support for .file symbols

Add support for emitting .file records.  This is mostly a quality of
implementation change (more complete support for COFF file emission) that was
noticed while working on COFF file emission for Windows on ARM.

A .file record is emitted as a symbol with storage class FILE (103) and the name
".file".  A series of auxiliary format 4 records follow which contain the file
name.  The filename is stored as an ANSI string and is padded with NULL if the
length is not a multiple of COFF::SymbolSize (18).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206355 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/WinCOFFStreamer.cpp

@@ -266,8 +266,7 @@ void WinCOFFStreamer::EmitTBSSSymbol(const MCSection *Section, MCSymbol *Symbol,
 }
 
 void WinCOFFStreamer::EmitFileDirective(StringRef Filename) {
-  // Ignore for now, linkers don't care, and proper debug
-  // info will be a much large effort.
+  getAssembler().addFileName(Filename);
 }
 
 // TODO: Implement this if you want to emit .comment section in COFF obj files.