[clangd] Rename some protocol field to lower case

Summary:
Also fixes a GCC compilation error.

Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>

Subscribers: klimek, ilya-biryukov, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D43411

llvm-svn: 325409