[clangd] Log rather than assert on bad UTF-8.

Summary:
I don't love this behavior, but it prevents crashing when indexing boost
headers, and I can't think of a better practical alternative.

Fixes https://reviews.llvm.org/D81530

Based on a patch by AnakinZheng!

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81530