Previously `OpExtInst` and `OpExtInstImport` was hard-coded to only support `GLSL.std.450`.
We will want to support `OpenCL.Debug.100`, so put in the plumbing to properly support other extensions.

Bug: b/145351270
Change-Id: I60fbb8c45bb57b747067437641676946129b1251
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39885Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>