Refactor getMinPlatformVersion
The definitions of how version numbers were extracted from target
triples split between the minimum platform version and for determining
the minimum inlining version.
This resulted in inlinable and transparent functions not being imported
correctly on non-Apple platforms where the version number is retained as
part of the target triple.
Specifically, `_checkExpectedExecutor` was found in the module, but
didn't have the appropriate availability version assigned, resulting in
it failing to import and the compiler silently omitting the check in
SILGen when compiling for FreeBSD.
This patch refactors the implementation of `getMinPlatformVersion` into
a separate function that is used in both places so that they cannot get
out of sync again.
Note: This changes how Windows is handled. getMinPlatformVersion
returned an empty version number for Windows, while the availability
implementation returned the OS version number. This makes both
consistently return the OS version number.
