[gold-plugin] Disable name for values other than GlobalValue

Summary:
Applies Mehdi's optimization (r263086) to disable value names other than
for GlobalValues to LTO/ThinLTO performed via the gold-plugin, in the
same manner as it is applied in libLTO.

Reviewers: rafael, joker-eph

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19269

llvm-svn: 266871