[SPIR-V] Map IR function pointers to registers in ModuleAnalysis

SPIRVModuleAnalysis collects module and external function registers
(usually result of OpFunction) for use when emitting OpFunctionCall.
This patch makes the mapping between the functions and registers using
pointers (instead of name strings) to ensure anonymous functions and
calls can be resolved properly.

Differential Revision: https://reviews.llvm.org/D140548