[ARM] Follow-up to fix crash "-target arm -mcpu=generic", without "-march="

Fix of dangling StringRef after temporary std::string is destroyed

Follow-up to: http://reviews.llvm.org/rL248479

Reviewers: alexfh

Subscribers: cfe-commits
llvm-svn: 248480
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 4061598..218d121 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -6165,7 +6165,7 @@
                                        const llvm::Triple &Triple) {
   unsigned ArchKind;
   if (CPU == "generic") {
-    StringRef ARMArch = tools::arm::getARMArch(Arch, Triple);
+    std::string ARMArch = tools::arm::getARMArch(Arch, Triple);
     ArchKind = llvm::ARM::parseArch(ARMArch);
     if (ArchKind == llvm::ARM::AK_INVALID)
       // In case of generic Arch, i.e. "arm",