Metal backend will reuse Vulkan's GlslangWrapper code.
The Metal backend will use this code to translate glsl to spirv then
cross compile to Metal Shading Language using spirv-cross.
So the source code of GlslangWrapper should be moved outside vulkan folder.

Bug: angleproject:2634
Change-Id: I208062daf0d77756c9d32cfdab925b7dfdf83e05
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1858042Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>