[clangd] Another improvement for std include mapping.

Summary:
Improve the way of checking a symbol name is in the first cell. The previous way
is not very robost for cases where a cell lists multiple symbols (e.g. int8_t).

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62575

llvm-svn: 361951