[SPIRV] Fix bug in emitting GLSL ext inst names

Lookup extended instruction numbers in the given instruction set so that
correct names are now emitted for GLSL.std.450 instructions as well as
OpenCL.std.

Add a single test to verify correct abs intrinsic names are emitted when
targetting logical SPIR-V.

Depends on D156424

Differential Revision: https://reviews.llvm.org/D159227