Some function types are special to us, so add an enum and determinte the
function kind once when creating the function, instead of looking at the
Decl every time we need the information.
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`.
