[clangd] Fix a crash when presenting values for Hover

Summary:
We should pass the expression type, not a variable type when printing
the resulting value. Variable type may be different from what the
pretty-printing function expects, e.g. have references.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65655

llvm-svn: 367687