[clangd] Add .cu files to VSCode extension

Summary:
clangd should be able to handle those with a proper compilation
database. However, users using 'nvcc' might start seeing spurious errors
in '.cu' files after this change.

My plan is to land and release this, but be ready to revert in
case of negative user feedback.

Reviewers: hokein

Reviewed By: hokein

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59759

llvm-svn: 356916