[Sema] Fix crash on __fp16 parameters in template instantiations

Fixes #61441.

Currently, Clang stores `nullptr` in the parameter lists inside
`FunctionProtoTypeLoc` if `__fp16` is used without pointer qualifiers.

Any code path that calls `Declarator::setInvalidType()` before
`GetFullTypeForDeclarator` will lead to the same problem downstream.

The relevant code is:
```cpp
if (D.isInvalidType())
  return Context.getTrivialTypeSourceInfo(T);

return GetTypeSourceInfoForDeclarator(state, T, TInfo);
```

`GetTypeSourceInfoForDeclarator` sets the parameter `Decl`, but we can't
call it when `isInvalidType() == true` as this causes other assertion
failures that seem harder to fix.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D146426