rdar://149985633
Using at least Int8 here allows LLVM to apply more optimizations, reducing code size, avoiding stack allocations and as a result often eliminating complete stack frames.
Use the generic type lowering algorithm described in
"docs/CallingConvention.rst#physical-lowering" to map from IRGen's explosion
type to the type expected by the ABI.
Change IRGen to use the swift calling convention (swiftcc) for native swift
functions.
Use the 'swiftself' attribute on self parameters and for closures contexts.
Use the 'swifterror' parameter for swift error parameters.
Change functions in the runtime that are called as native swift functions to use
the swift calling convention.
rdar://19978563
This prevents the linker from trying to emit relative relocations to locally-defined public symbols into dynamic libraries, which gives ld.so heartache.
If the payload has insufficient extra inhabitants and there's more than one empty case that requires spilling extra tag bits, we need to check the payload part of the value to discriminate them.
