FreeBSD: No availability OS version

Transparent and inlinable declarations had the OS version availability
applied to them. This broke some declarations coming from the standard
library, specifically `_checkExpectedExecutor`. Rather than crashing,
SILGen silently omits the check, resulting in failing tests, and
unchecked code. FreeBSD is not one of the platforms where availability
is set up, so clear the availability version number like what is done
for all non-Apple platforms.

Author: etcwilde

lib/Frontend/CompilerInvocation.cpp

@@ -64,6 +64,12 @@ getVersionTuple(const llvm::Triple &triple) {
     triple.getMacOSXVersion(OSVersion);
     return OSVersion;
   }
+  if (triple.isOSFreeBSD()) {
+    // FreeBSD keeps the version number as part of the version tuple.
+    // We should not use this triple for availability checking, which only works
+    // with Apple platforms.
+    return llvm::VersionTuple();
+  }
   return triple.getOSVersion();
 }