[clangd] Mark colon as a safe character when percent-encoding.

Summary: Also change output of percent-encoding to use upper-case letters.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D53016

llvm-svn: 344033