[clangd] SymbolLocation only covers symbol name.

Summary:
* Change the offset range to half-open, [start, end).
* Fix a few fixmes.

Reviewers: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D43182

llvm-svn: 324992