Pass llvm::Triple objects by const reference.

Copying isn't cheap as it contains a std::string.

llvm-svn: 202880
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 83ba05b..9232a5d 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -2757,7 +2757,7 @@
 /// a target-triple directory in the library and header search paths.
 /// Unfortunately, this triple does not align with the vanilla target triple,
 /// so we provide a rough mapping here.
-static std::string getMultiarchTriple(const llvm::Triple TargetTriple,
+static std::string getMultiarchTriple(const llvm::Triple &TargetTriple,
                                       StringRef SysRoot) {
   // For most architectures, just use whatever we have rather than trying to be
   // clever.