[clangd] Include expression in DecltypeTypeLoc sourcerange while building SelectionTree

Summary:
Currently AST only contains the location for `decltype` keyword,
therefore we were skipping expressions inside decltype while building selection
tree.

This patch extends source range in such cases to contain the expression as well.
A proper fix would require changes to Sema and DecltypeTypeLoc to contain these
location information.

Fixes https://github.com/clangd/clangd/issues/250.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72594