Initial support for compiler AST validation This is to be able to catch issues introduced through AST manipulation earlier than shader compilation time, improving debuggability. Bug: angleproject:2733 Change-Id: Ic57bc72f2ab438e60f32553d602074f3d72cc4f5 Reviewed-on: https://chromium-review.googlesource.com/c/1199922 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>