[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

llvm-svn: 353888