[clangd] Avoid scanning up to end of file on each comment!

Assigning char* (pointing at comment start) to StringRef was causing us
to scan the rest of the source file looking for the null terminator.

This seems to be eating about 8% of our *total* CPU!

While fixing this, factor out the common bits from the two places we're
parsing IWYU pragmas.

Differential Revision: https://reviews.llvm.org/D135314