[gn build] Separate debug and optimization settings
This patch adds an is_optimized variable, orthogonal to is_debug, to allow for a gn analogue to RelWithDebInfo builds.
As part of this we'll want to explicitly enable GC+ICF, for the sake of is_debug && is_optimized builds. The flags normally default to true except that if you pass /DEBUG they default to false.
Differential Revision: https://reviews.llvm.org/D58075