[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
Patch by Peiyuan Song!
Differential Revision: https://reviews.llvm.org/D55407