[ExecutionEngine] Fix MCJIT::addGlobalMapping.

This patch fixes MCJIT::addGlobalMapping by changing the implementation of the
ExecutionEngineState class. The new implementation maintains a bidirectional
mapping between symbol names (std::strings) and addresses (uint64_ts), rather
than a mapping between Value*s and void*s.

This has fix has been made for backwards compatibility, however the strongly
preferred way to resolve unknown symbols is by writing a custom
RuntimeDyld::SymbolResolver (formerly RTDyldMemoryManager) and overriding the
findSymbol method. The addGlobalMapping method is a hangover from the legacy JIT
(which has was removed in 3.6), and may be deprecated in a future release as
part of a clean-up of the ExecutionEngine interface.

Patch by Murat Bolat. Thanks Murat!

llvm-svn: 233747