[test] Tidy up newly-added clang_rt Driver tests to be a bit clearer

Per feedback from David U.

Author: jrose-apple

test/Driver/linker-clang_rt.swift

@@ -1,6 +1,5 @@
-// REQUIRES: OS=macosx
-// Note: This is really about the /host/ environment, but since there are RUN
-// lines for multiple targets anyway it doesn't make a huge difference.
+// Make sure that the platform-appropriate clang_rt library (found relative to
+// the compiler) is included when using Swift as a linker (with Apple targets).
 
 // We use hard links to make sure the Swift driver really thinks it's been
 // moved.
@@ -27,6 +26,9 @@
 // RUN: %t/bin/swiftc -driver-print-jobs -target i386-apple-watchos2 %S/../Inputs/empty.swift | %FileCheck -check-prefix CHECK -check-prefix WATCHOS %s
 // RUN: %t/bin/swiftc -driver-print-jobs -target armv7k-apple-watchos2 %S/../Inputs/empty.swift | %FileCheck -check-prefix CHECK -check-prefix WATCHOS %s
 
+// Clean up the test executable because hard links are expensive.
+// RUN: rm -f %t/bin/swiftc
+
 // CHECK: bin/ld{{"? }}
 // CHECK-NO-RUNTIME-NOT: libclang_rt
 // CHECK-MACOS-SAME: {{[^ ]+/lib/clang/darwin/libclang_rt.osx.a}}

validation-test/Driver/Inputs/clang_rt-helper.h

@@ -1,7 +1,7 @@
 #include <stdbool.h>
 
-static inline bool testNewOS() {
-  if (__builtin_available(macOS 10.12, *)) {
+static inline bool isRunningOnFairlyRecentOS() {
+  if (__builtin_available(macOS 10.12, iOS 10, tvOS 10, watchOS 3, *)) {
     return true;
   } else {
     return false;

validation-test/Driver/clang_rt.swift

@@ -4,7 +4,7 @@
 
 // Just make sure we can build and link successfully.
 
-if testNewOS() {
+if isRunningOnFairlyRecentOS() {
   print("new!")
 } else {
   print("old...")