[clangd] Simplify the callside of URI::resolve, NFC.

[clangd] Simplify the callside of URI::resolve, NFC.

Summary:
- Add a overrloded URI::resolve, which accepts a string URI;
- also fixed some callside that don't check the error;

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67916

llvm-svn: 372617