Fix usage in TableGen of getValueAsString

Record::getValueAsString returns a stringref to an interned
string (apparently had been changed since most of tablegen was
written). In this patch, I audited the usage of getValueAsString
to find places where we can trivially stop storing 'std::string' and instead
keep the stringref.

There was one instance where an unnecessary 'stringstream' was being used,
so that has been removed as well to unblock the stringref replacing string fix.

Differential Revision: https://reviews.llvm.org/D38979

llvm-svn: 315956