Add support for emitting a string attribute.

Attributes that are strings are typically target-dependent attributes. They are
of this form in the IR:

   "attr"
   "attr" = "val"


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174090 91177308-0d34-0410-b5e6-96231b3b80d8