[clangd] Change PreambleOnlyAction with content truncation

Summary:
Lexing until the token location is past preamble bound could be wrong
in some cases as preprocessor lexer can lex multiple tokens in a single call.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79426