Avoid extra back reference key lookup in msmangler

Avoid a second key lookup when the back reference key is going to be
inserted in the StringMap. The string lookups in the msmangler are the
main responsible for the huge overhead when compared to the itanium
mangler. This patch makes a small but noticeable improvement.

Reviewed by: rnk

Differential Revision: http://reviews.llvm.org/D4130

Patch by Agustín Bergé!