[clangd] in VSCode client, filter extensions properly and only accept file: URIs

Summary:
The filtering wasn't previously working as intended - the string list is
interpreted as a list of editor modes, not file extensions.
(It happens to mostly work as "c" and "cpp" are the names of modes, but we're
missing objective-c)

The file: restriction is new - clangd needs to be able to convert URI<->path
in order to determine how to build.

Reviewers: hokein

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41343

llvm-svn: 320972