[COFF] Don't write long section names for sections that will be mapped at runtime

Sections that will be mapped at runtime will only have the short
section name available, since the string table it points into isn't
mapped. Therefore prefer truncating those names over writing a
long name that is unavailable at runtime.

This allows libunwind to find the .eh_frame section at runtime even
if the module was built with debug info enabled.

Differential Revision: https://reviews.llvm.org/D40025

llvm-svn: 318391