The methods are now attached to CodegenCx instead of Type

Moved common enums to common

Prelude to using associated types in traits rather than type parameters

Generalized operand.rs#nontemporal_store and fixed tidy issues

Generalized operand.rs#nontemporal_store's implem even more
With a BuilderMethod trait implemented by Builder for LLVM

Cleaned builder.rs : no more code duplication, no more ValueTrait

Full traitification of builder.rs

If LLVM 7 is used, generate memcpy/memmove with differing
src/dst alignment. I've added new FFI functions to construct
these through the builder API, which is more convenient than
dealing with differing intrinsic signatures depending on the
LLVM version.

This commit updates rustc to wait for all codegen threads to exit before
allowing the main thread to exit. This is a stab in the dark to fix the
mysterious segfaults appearing on #55238, and hopefully we'll see
whether this actually fixes things in practice...

While we still have to support LLVM 4.0 for Emscripten, we can
drop checks for LLVM >= 4.0 and < 4.0.