Getting rid of a string literal in favor of the generalized diagnostic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198050 91177308-0d34-0410-b5e6-96231b3b80d8
Simplifying some diagnostics so that they don't need to work with StringRefs. No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198051 91177308-0d34-0410-b5e6-96231b3b80d8
s/getter_setter/accessor No functional changes intended.

Thanks to Alp Toker for the naming suggestion!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198052 91177308-0d34-0410-b5e6-96231b3b80d8