[Sema] Fix assertion hit with #pragma weak.

rdar://16264844

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203372 91177308-0d34-0410-b5e6-96231b3b80d8
Remove uses of SmallString::equals in favor of SmallVectorImpl<char>'s operator==

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203373 91177308-0d34-0410-b5e6-96231b3b80d8