Use None instead of an explicit constructor.

And remove a needless 'const' since ArrayRef is immutable.
Both as suggested by dblaikie on cfe-commits.

llvm-svn: 247995
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 8ec636a..f231282 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -1171,7 +1171,7 @@
 /// triple.
 void Generic_GCC::GCCInstallationDetector::init(
     const Driver &D, const llvm::Triple &TargetTriple, const ArgList &Args,
-    const ArrayRef<std::string> ExtraTripleAliases) {
+    ArrayRef<std::string> ExtraTripleAliases) {
   llvm::Triple BiarchVariantTriple = TargetTriple.isArch32Bit()
                                          ? TargetTriple.get64BitArchVariant()
                                          : TargetTriple.get32BitArchVariant();
diff --git a/clang/lib/Driver/ToolChains.h b/clang/lib/Driver/ToolChains.h
index 203e30b..f14fc3e7 100644
--- a/clang/lib/Driver/ToolChains.h
+++ b/clang/lib/Driver/ToolChains.h
@@ -102,8 +102,7 @@
     GCCInstallationDetector() : IsValid(false) {}
     void init(const Driver &D, const llvm::Triple &TargetTriple,
               const llvm::opt::ArgList &Args,
-              const ArrayRef<std::string> ExtraTripleAliases =
-                  ArrayRef<std::string>());
+              ArrayRef<std::string> ExtraTripleAliases = None);
 
     /// \brief Check whether we detected a valid GCC install.
     bool isValid() const { return IsValid; }