[COFF] Map truncated .eh_frame section name

PE/COFF sections can have section names truncated to 8 chars, in order to
have the name available at runtime. (The string table, where long untruncated
names are stored, isn't loaded at runtime.)

This allows various llvm tools to dump the .eh_frame section from such
executables.

Patch by Peiyuan Song!

Differential Revision: https://reviews.llvm.org/D55407

llvm-svn: 348708