swift234

Emit a warning when using unknown arch/os build configurations

'arch' and 'os' build configurations with valid identifiers as
arguments, but which are unknown to the compiler, will cause the
compiler to silently skip over that code as it has an inactive clause.
Emit a diagnostic, but not an error so as not to inadvertantly break
code that may be in a compiler without knowledge of a particular
operating system or architecture.

rdar://problem/22052176

Swift SVN r32219