This adds the swiftMSVCRT module which is similar in spirit to swiftGlibc and
swiftDarwin, exposing the Microsoft C Runtime library to swift. Furthermore,
disable pieces of the standard library which are not immediately trivially
portable to Windows. A lot of this functionality can still be implemented and
exposed to the user, however, this is the quickest means to a PoC for native
windows support.
As a temporary solution, add a -DCYGWIN flag to indicate that we are building
for the cygwin windows target. This allows us to continue supporting the cygwin
environment whilst making the windows port work natively against the windows
environment (msvc). Eventually, that will hopefully be replaced with an
environment check in swift.
The general rule here is that something needs to be SWIFT_CC(swift)
if it's just declared in Swift code using _silgen_name, as opposed to
importing something via a header.
Of course, SWIFT_CC(swift) expands to nothing by default for now, and
I haven't made an effort yet to add the indirect-result / context
parameter ABI attributes. This is just a best-effort first pass.
I also took the opportunity to shift a few files to just implement
their shims header and to demote a few things to be private stdlib
interfaces.
