Recommit 211309 (StringMap::insert), reverted in 211328 due to issues with...

Recommit 211309 (StringMap::insert), reverted in 211328 due to issues with private, but non-deleted, move members.

Certain versions of GCC (~4.7) couldn't handle the SFINAE on access
control, but with "= delete" (hidden behind a macro for portability)
this issue is worked around/addressed.

Patch by Agustín Bergé

llvm-svn: 211525