[clangd] Make SymbolLocation => bool conversion explicitly.

Summary:
The implicit bool conversion could happen superisingly, e.g. when
checking `if (Loc1 == Loc2)`, the compiler will convert SymbolLocation to
bool before comparing (because we don't define operator `==` for SymbolLocation).

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D49657

llvm-svn: 338517