Introduce CodeSectionINTEL storage class (#2728)

This storage class is used for function pointers. It's added as based on cl_intel_function_pointers specification, it is not guaranteed that sizeof(void(*)(void) == sizeof(void *) - to allow consumers use this fact, we cannot say that function pointer belongs to the same storage class as data pointers.

It wasn't added during initial implementation, now it's time to fill this gap. As it would be a breaking change its generation is added only under -spirv-emit-function-ptr-addr-space option. Also SPIR-V consumer may pass this option during reverse translation to get new address space even in a case, when OpConstantFunctionPointerINTEL doesn't reside in CodeSectionINTEL storage class.

Expected behavior:

No option is passed to the forward translation stage and function pointers are in addrspace(9):
no CodeSectionINTEL storage class in SPIR-V
The option is passed to the forward translation stage and function pointers are in addrepace(9):
CodeSectionINTEL storage class is generated
No option is passed to the reverse translation stage: function pointers are in private address space
The option is passed to the reverse translation stage: function pointers are in addrspace(9)
Spec:
https://github.com/intel/llvm/blob/sycl/sycl/doc/design/spirv-extensions/SPV_INTEL_function_pointers.asciidoc

The previous approach: #1392

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@46c9eb9ea9b4c6f

Author: MrSidims

llvm-spirv/lib/SPIRV/SPIRVReader.cpp

@@ -3073,7 +3073,7 @@ void SPIRVToLLVM::transFunctionAttrs(SPIRVFunction *BF, Function *F) {
   });
 }
 
-Function *SPIRVToLLVM::transFunction(SPIRVFunction *BF) {
+Function *SPIRVToLLVM::transFunction(SPIRVFunction *BF, unsigned AS) {
   auto Loc = FuncMap.find(BF);
   if (Loc != FuncMap.end())
     return Loc->second;

llvm-spirv/lib/SPIRV/SPIRVReader.h

@@ -103,7 +103,7 @@ class SPIRVToLLVM : private BuiltinCallHelper {
   void transAuxDataInst(SPIRVExtInst *BC);
   std::vector<Value *> transValue(const std::vector<SPIRVValue *> &,
                                   Function *F, BasicBlock *);
-  Function *transFunction(SPIRVFunction *F);
+  Function *transFunction(SPIRVFunction *F, unsigned AS = SPIRAS_Private);
   void transFunctionAttrs(SPIRVFunction *BF, Function *F);
   Value *transBlockInvoke(SPIRVValue *Invoke, BasicBlock *BB);
   Instruction *transWGSizeQueryBI(SPIRVInstruction *BI, BasicBlock *BB);