[clang-cl] Handle some pragma alloc_text corner cases handled by MSVC

MSVC's pragma alloc_text accepts a function that was redeclared in
a non extern-C context if the previous declaration was in an extern-C
context. i.e.

```
extern "C" { static void f(); }
static void f();
```

MSVC's pragma alloc_text also rejects non-functions.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D128649