The runtime doesn't really need Compiler.h. It just needs some
visibility macros which can be inlined here instead of pulling
the whole heavyweight header (including its transitive closure,
llvm-config.h). This is becoming more important now that Compiler.h
includes C++ headers (namely, <new>), and swift/Runtime/Config.h
can be included from C or Objective-C files (causing build failures).
<rdar://problem/35860874>
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
