Use a std::string rather than a dynamically allocated char* buffer.

Use a std::string rather than a dynamically allocated char* buffer.

This affords us to use std::string's allocation routines and use the destructor
for the memory management. Switching to that also means that we can use
operator==(const std::string&, const char *) to perform the string comparison
rather than resorting to libc functionality (i.e. strcmp).

Patch by Saleem Abdulrasool!

Differential Revision: http://llvm-reviews.chandlerc.com/D230

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171042 91177308-0d34-0410-b5e6-96231b3b80d8