ARM: convert isTargetIOS checks to isTargetDarwin.

The distinction is mostly useful in the front-end. By the time we get here,
there are very few situations where we actually want different behaviour for
Darwin and IOS (in fact Darwin mostly just exists in a few tests). So this
should reduce any surprising weirdness for anyone using it.

No functional change on anything anyone actually cares about.

llvm-svn: 224035
diff --git a/llvm/lib/Target/ARM/ARMSubtarget.cpp b/llvm/lib/Target/ARM/ARMSubtarget.cpp
index e53cef1..fa65375 100644
--- a/llvm/lib/Target/ARM/ARMSubtarget.cpp
+++ b/llvm/lib/Target/ARM/ARMSubtarget.cpp
@@ -215,7 +215,7 @@
 
 void ARMSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
   if (CPUString.empty()) {
-    if (isTargetIOS() && TargetTriple.getArchName().endswith("v7s"))
+    if (isTargetDarwin() && TargetTriple.getArchName().endswith("v7s"))
       // Default to the Swift CPU when targeting armv7s/thumbv7s.
       CPUString = "swift";
     else