FunctionDecl::getBuiltinID() is surprisingly slow and we tend to call it
quite a bit, especially when interpreting builtin functions. Caching the
BuiltinID here reduces the time I need to compile the
floating_comparison namespace from builtin-functions.cpp from 7.2s to
6.3s locally.
I started out by adding a new pointer type for blocks, and I was fully
prepared to compile their AST to bytecode and later call them.
... then I found out that the current interpreter doesn't support
calling blocks at all. So we reuse `Function` to support sources other
than `FunctionDecl`s and classify `BlockPointerType` as `PT_FnPtr`.
