[CIR][CodeGen] Replace uses of global with alias

[tommymcm]

Minimized test case:

class A {
public:
  A();
} A;
A::A() {}

Failed assertion:

clangir/clang/lib/CIR/CodeGen/CIRGenModule.cpp

Line 2479 in aeac352

llvm_unreachable("NYI");

Flags: -fclangir -O2