COFF: add support for .file symbols

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