Support of symbol references in CIRGenExprConst

[yugr]

Some parts of CIRGenExprConst.cpp assumes that attributes generated for RHS of assignments are TypedAttr's e.g.

  auto typedC = llvm::dyn_cast<mlir::TypedAttr>(C);                                                                                                                                                        
  if (!typedC)                                                                  
    llvm_unreachable("this should always be typed");                            
  return typedC;                                                                

But in some cases SymbolRefAttr could be generated as well e.g. in

$ cat tmp.c
typedef struct {
  char *name;
} A;

A foo = {"1"};

$ /home/huawei/src/clangir/build/bin/clang -fclangir-enable -S tmp.c
this should always be typed
UNREACHABLE executed at /home/huawei/src/clangir/clang/lib/CIR/CodeGen/CIRGenExprConst.cpp:1428!
...
#13 0x00007fa8faecf572 cir::ConstantEmitter::tryEmitPrivate(clang::Expr const*, clang::QualType) /home/huawei/src/clangir/clang/lib/CIR/CodeGen/CIRGenExprConst.cpp:1429:10

Perhaps GlobalViewAttr should be used instead of SymbolRefAttr in such cases?

[bcardosolopes]

Thanks for reporting the crash. Yes, those should be typed, for some context, since this logic is earlier than when MLIR split types out of attributes, it could be we didn't handle such path properly once MLIR changed.

Anyways, I haven't looked in depth yet, but either your suggestion or somehow via cir.get_global

[yugr]

Fixed with #243.