I use glslang -E as a preprocessor before minifying. This can result in inputs to the minifier such as

#version 450
#extension GL_GOOGLE_include_directive : enable
#line 1 "linedir2.glsl"
void foo() { }
#line 4 "linedir.glsl"
 void main() { }

#line is useful for tracking original line numbers in case of error/warning messages, but this is undesirable when optimizing for size.
#extension GL_GOOGLE_include_directive : enable is needed on the input to turn on #include support in glslang, but is useless on the output since no GL implements it and any includes have already been processed. This is arguably a glslang bug, but it is easy to handle within the minifier.

Thanks!
As you can see, the preprocessor is very limited. I didn't know if anyone was using it in the current state.