Report an error when -m<os>-version-min= does not specify a version.

Currently if you use -mmacosx-version-min or -mios-version-min without
specifying a version number, clang silently sets the minimum version to
"0.0.0". This is almost certainly not what was intended, so it is better
to report it as an error. rdar://problem/20433945

llvm-svn: 234270
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 5b01b1c..07a5e42 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -2152,7 +2152,7 @@
 
   Major = Minor = Micro = 0;
   if (*Str == '\0')
-    return true;
+    return false;
 
   char *End;
   Major = (unsigned) strtol(Str, &End, 10);