Add a copy constructor to StringMap

There is code under review that requires StringMap to have a copy constructor,
and this makes StringMap more consistent with our other containers (like
DenseMap) that have copy constructors.

Differential Revision: http://reviews.llvm.org/D18506

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264906 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed