Remove a FIXME about truncated section names

If the linker chose to store the full section name instead of truncating
it, this field doesn't contain a truncated name, but an offset into
the string table of the binary. The string table isn't loaded/mapped
into memory during runtime though, so it's not possible to read the
full section name, unless we try to locate the DLL/EXE on disk that
the HMODULE corresponds to and load that manually.

Due to this, lld now always prefers writing a truncated section name
for sections that will be mapped at runtime, even when debug info is
enabled.

Differential Revision: https://reviews.llvm.org/D39918

llvm-svn: 318446