swift234

stdlib: be more DLL friendly on Windows

When the standard library is built dynamically on COFF targets, the public
interfaces must be decorated in order to generate a proper DLL which can be
confused by the dependent libraries.  When the exported interface is used, it
must be indirectly addressed.  This can be done manually in code or the MS
extension of `__declspec(dllimport)` may be used to indicate to the compiler
that this symbol be addressed indirectly.  This permits building more pieces of
the standard library dynamically on Windows.