PECOFF: Use the string table for long section names in EXEs/DLLs

Normally, PE files have section names of eight characters or less.
However, this is problematic for DWARF because DWARF section names are
things like .debug_aranges.

Instead of truncating the section name, redirect the section name into
the string table.

Differential Revision: http://reviews.llvm.org/D6104

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221212 91177308-0d34-0410-b5e6-96231b3b80d8