Add more robustness - use PyString_CheckExact(pvalue) to check whether pvalue...

Add more robustness - use PyString_CheckExact(pvalue) to check whether pvalue is a Python string before
calling PyString_AsString(pvalue).  Similar to http://llvm.org/viewvc/llvm-project?rev=146584&view=rev.

llvm-svn: 146606