[PDB] Correctly link S_FILESTATIC records.

This is not a record type that clang currently generates,
but it is a record that is encountered in object files generated
by cl.  This record is unusual in that it refers directly to
the string table instead of indirectly to the string table via
the FileChecksums table.  Because of this, it was previously
overlooked and we weren't remapping the string indices at all.
This would lead to crashes in MSVC when trying to display a
variable whose debug info involved an S_FILESTATIC.

Original bug report by Alexander Ganea

Differential Revision: https://reviews.llvm.org/D41718

llvm-svn: 321883