[MCJIT] Add a FindGlobalVariableNamed utility

Summary: This adds FindGlobalVariableNamed to ExecutionEngine
(plus implementation in MCJIT), which is an analog of
FindFunctionNamed for GlobalVariables.

Reviewers: lhames

Reviewed By: lhames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10421

llvm-svn: 240202