Fix a bug PlatformDarwin::SDKSupportsModule.

This fixes a bug PlatformDarwin::SDKSupportsModule introduced by
https://reviews.llvm.org/D47889.  VersionTuple::tryParse() can deal
with an optional third (micro) component, but the parse will fail when
there are extra characters after the version number (e.g.: trying to
parse the substring "12.0.sdk" out of "iPhoneSimulator12.0.sdk" fails
after that patch).  Fixed here by stripping the ".sdk" suffix first.

(Part of) rdar://problem/45041492

Differential Revision https://reviews.llvm.org/D53677

llvm-svn: 345274