[clangd] Avoid unexpected desugaring in isSugaredTemplateParameter

This is a follow-up to D151785, addressing https://github.com/clangd/clangd/issues/1703.

The previous approach of peeling pointer types during a traversal
using getPointeeType might have produced unexpected results; since
the method would implicitly desugar the type if the type being passed
in could not be cast to a Pointer-like Type.

Reviewed By: nridge

Differential Revision: https://reviews.llvm.org/D156300