[llvm][utils] Fix SmallVector formatter when type is a pointer

Previously types such as `SmallVector<clang::Attr, 4> *` would
trigger the assertion inside the `SmallVector` formatter:
```
assert self.type_size != 0
```
This happens because `the_type.GetTemplateArgumentType(0)` returns
`None` (since `the_type` is a pointer to the `SmallVector`).

This patch dereferences `the_type` if it's a pointer type. We do this
for references already.

Differential Revision: https://reviews.llvm.org/D157961