[clang] [CodeGen] Add CodeGenModule::forgetDecl to discard cached mangled-name state (PR #234493)

[via cfe-commits]

SahilPatidar wrote:

Yes, I think this way:
```cpp
extern "C" int printf(const char *, ...);
static int sf(int x);
static int sf(int x) { return x + 1; }
%undo
auto sfr = printf("sf=%d\n", sf(10));
// CHECK-NOT: sf=11
// CHECK: JIT session error: Symbols not found
```

https://github.com/llvm/llvm-project/pull/234493