[clangd] Fix a hover crash on unsigned 64bit value

This patch adapts to D140059, which makes an assumption that the
caller of `APSInt::getExtValue` promises no narrowing conversion
happens, i.e., from unsigned int64 to signed int64.

It also fixes clangd/clangd#1557.

Reviewed By: nridge

Differential Revision: https://reviews.llvm.org/D146874