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);
diff --git a/clang/test/Driver/darwin-version.c b/clang/test/Driver/darwin-version.c
index 8e90927..3e7496b 100644
--- a/clang/test/Driver/darwin-version.c
+++ b/clang/test/Driver/darwin-version.c
@@ -30,6 +30,9 @@
 // RUN: %clang -target x86_64-apple-macosx -mmacosx-version-min=10.10 -c %s -### 2>&1 | \
 // RUN:   FileCheck --check-prefix=CHECK-VERSION-OSX10 %s
 // CHECK-VERSION-OSX10: "x86_64-apple-macosx10.10.0"
+// RUN: %clang -target x86_64-apple-macosx -mmacosx-version-min= -c %s -### 2>&1 | \
+// RUN:   FileCheck --check-prefix=CHECK-VERSION-MISSING %s
+// CHECK-VERSION-MISSING: invalid version number
 
 // Check environment variable gets interpreted correctly
 // RUN: env MACOSX_DEPLOYMENT_TARGET=10.5 \