[clangd] support expanding `decltype(expr)`

Enable the existing tweak `ExpandAutoType` to expand
`decltype(expr)`, e.g.

```
decltype(0) i;
```

will expand to

```
int i;
```

Therefore, rename the tweak `ExpandAutoType` to `ExpandDeducedType`.

This patch also fixes some nits,

* avoid replacing reference to a function
* avoid replacing array types and reference to an array

Fixes https://github.com/clangd/clangd/issues/1456

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D141226