[MCJIT] Start Stringref-izing the ExecutionEngine interface.

More methods to follow.

Using StringRef allows us the EE interface to work with more string types
without forcing construction of std::strings.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217794 91177308-0d34-0410-b5e6-96231b3b80d8