Fix a use-after-free bug found by ASan. You can't assign a temporary
std::string to a StringRef. Moreover, the method being called accepts
a Twine to simplify these patterns.

Fixes this ASan failure:
==6312== ERROR: AddressSanitizer: heap-use-after-free on address 0x7fd558b1af58 at pc 0xcb7529 bp 0x7fffff572080 sp 0x7fffff572078
READ of size 1 at 0x7fd558b1af58 thread T0
    #0 0xcb7528 .../llvm/include/llvm/ADT/StringRef.h:192 llvm::StringRef::operator[]()
    #1 0x1d53c0a .../llvm/include/llvm/ADT/StringExtras.h:128 llvm::HashString()
    #2 0x1d53878 .../llvm/lib/Support/StringMap.cpp:64 llvm::StringMapImpl::LookupBucketFor()
    #3 0x1b6872f .../llvm/include/llvm/ADT/StringMap.h:352 llvm::StringMap<>::GetOrCreateValue<>()
    #4 0x1b61836 .../llvm/lib/MC/MCContext.cpp:109 llvm::MCContext::GetOrCreateSymbol()
    #5 0xe9fd47 .../llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp:154 (anonymous namespace)::ARMELFStreamer::EmitMappingSymbol()
    #6 0xea01dd .../llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp:133 (anonymous namespace)::ARMELFStreamer::EmitDataMappingSymbol()
    #7 0xe9f78b .../llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp:91 (anonymous namespace)::ARMELFStreamer::EmitBytes()
    #8 0x1b15d82 .../llvm/lib/MC/MCStreamer.cpp:89 llvm::MCStreamer::EmitIntValue()
    #9 0xcc0f9b .../llvm/lib/Target/ARM/ARMAsmPrinter.cpp:713 llvm::ARMAsmPrinter::emitAttributes()
    #10 0xcc0d44 .../llvm/lib/Target/ARM/ARMAsmPrinter.cpp:632 llvm::ARMAsmPrinter::EmitStartOfAsmFile()
    #11 0x14692ad .../llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp:162 llvm::AsmPrinter::doInitialization()
    #12 0x1bc4677 .../llvm/lib/VMCore/PassManager.cpp:1561 llvm::FPPassManager::doInitialization()
    #13 0x1bc4990 .../llvm/lib/VMCore/PassManager.cpp:1595 llvm::MPPassManager::runOnModule()
    #14 0x1bc55e5 .../llvm/lib/VMCore/PassManager.cpp:1705 llvm::PassManagerImpl::run()
    #15 0x1bc5878 .../llvm/lib/VMCore/PassManager.cpp:1740 llvm::PassManager::run()
    #16 0xc3954d .../llvm/tools/llc/llc.cpp:378 compileModule()
    #17 0xc38001 .../llvm/tools/llc/llc.cpp:194 main
    #18 0x7fd557d6a11c __libc_start_main
0x7fd558b1af58 is located 24 bytes inside of 29-byte region [0x7fd558b1af40,0x7fd558b1af5d)
freed by thread T0 here:
    #0 0xc337da .../llvm/projects/compiler-rt/lib/asan/asan_new_delete.cc:56 operator delete()
    #1 0x1ee9cef .../libstdc++-v3/include/bits/basic_string.h:535 std::string::~string()
    #2 0xea01dd .../llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp:133 (anonymous namespace)::ARMELFStreamer::EmitDataMappingSymbol()
    #3 0xe9f78b .../llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp:91 (anonymous namespace)::ARMELFStreamer::EmitBytes()
    #4 0x1b15d82 .../llvm/lib/MC/MCStreamer.cpp:89 llvm::MCStreamer::EmitIntValue()
    #5 0xcc0f9b .../llvm/lib/Target/ARM/ARMAsmPrinter.cpp:713 llvm::ARMAsmPrinter::emitAttributes()
    #6 0xcc0d44 .../llvm/lib/Target/ARM/ARMAsmPrinter.cpp:632 llvm::ARMAsmPrinter::EmitStartOfAsmFile()
    #7 0x14692ad .../llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp:162 llvm::AsmPrinter::doInitialization()
    #8 0x1bc4677 .../llvm/lib/VMCore/PassManager.cpp:1561 llvm::FPPassManager::doInitialization()
    #9 0x1bc4990 .../llvm/lib/VMCore/PassManager.cpp:1595 llvm::MPPassManager::runOnModule()
    #10 0x1bc55e5 .../llvm/lib/VMCore/PassManager.cpp:1705 llvm::PassManagerImpl::run()
    #11 0x1bc5878 .../llvm/lib/VMCore/PassManager.cpp:1740 llvm::PassManager::run()
    #12 0xc3954d .../llvm/tools/llc/llc.cpp:378 compileModule()
    #13 0xc38001 .../llvm/tools/llc/llc.cpp:194 main
    #14 0x7fd557d6a11c __libc_start_main