[Symbolizer] Don't use PE symbol tables to override PDB symbols

Summary:
PE files are stripped by default, and only contain the names of exported
symbols.

The actual reason that we bother to do this override by default is
actually due to a quirk of the way -gline-tables-only is implemented, so
I phrased the check as "if we are symbolizing from dwarf, do the symtab
override".

This fixes lots of Windows ASan tests that I broke in r250582.

Reviewers: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14594

llvm-svn: 253051