Pull Compilation Fixits When Building

[dsanders11]

Pull any compilation errors from e build and turn them into VS Code diagnostic errors. Can get exact source range info from Clang if it runs with -fdiagnostics-print-source-range-info and can pull the quick fixes with -fdiagnostics-parseable-fixits.

[dsanders11]

Compilation errors are currently implemented and handled with problem matchers, which is nice. Getting the fixits won't be straightforward unfortunately since plumbing in that Clang compilation flag only when using the extension is tricky. Chromium has an apply_fixits.py script, and their advice for turning on the flag is to manually edit build/config/compiler/BUILD.gn which isn't going to work for this situation.