Composable profiles as code

[jbsolomon]

I'm not really experienced enough with C++ compiler architecture to have a thorough opinion on the design of this, but I think profiles should be something like composable rules or functions of the AST (?) that sufficiently knowledgeable users can create and compose. As I write this it strikes me that I'm just describing GCC/Clang plugins, but this should be portable.... pie-in-the-sky idea here but I hope it's worth mentioning!