Scavenge type parameters for use in translating opaque pointers. (#1512)

This change is incomplete, and will not properly convert every single LLVM IR
file to SPIR-V. However, it will convert many files. Areas that are known to
require more investigation:
* ptr-valued extractvalue, especially from cmpxchg instructions
* return values of functions
* llvm.*.annotation support
* function pointer extensions
* pointer types in structs

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@21fee92

Author: jcranmer-intel

llvm-spirv/lib/SPIRV/CMakeLists.txt

@@ -21,6 +21,7 @@ add_llvm_library(LLVMSPIRVLib
   SPIRVToOCL.cpp
   SPIRVToOCL12.cpp
   SPIRVToOCL20.cpp
+  SPIRVTypeScavenger.cpp
   SPIRVUtil.cpp
   SPIRVWriter.cpp
   SPIRVWriterPass.cpp