COFF: Stop emitting a non-standard COFF symbol table into PEs.

Now that our support for PDB emission is reasonably good, there is
no longer a need to emit a COFF symbol table.

Also fix a bug where we would fail to emit a string table for long
section names if /debug was not specified.

Differential Revision: https://reviews.llvm.org/D40189

llvm-svn: 318548