Add new ExecutionEngine::getGlobalValueAtAddress method, which can efficiently

Add new ExecutionEngine::getGlobalValueAtAddress method, which can efficiently
turn a memory address back into the LLVM global object that starts at that
address.  Note that this won't cause any additional datastructures to be built
for clients of the EE that don't need this information.

Also modified some code to not access the GlobalAddress map directly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10674 91177308-0d34-0410-b5e6-96231b3b80d8