Focus was on the front end (not SPIR-V), minus the grammar.
Reduces #ifdef count by around 320 and makes the web build 270K smaller,
which is about 90% the target size.
The grammar and scanner will be another step, as will the SPIR-V backend.
This makes heavy use of methods #ifdef'd to return false as a global way
of turning off code, relying on C++ DCE to do the rest.
Save about 100K.
N.B.: This is done by eliminating a function call, at a high level,
not by #ifdef'ing a bunch of code.
Also, removed no longer needed *_EXTENSION #ifdef in the code not
needed by GLSLANG_WEB.
