llvm-wrapper: pass std::string instead of StringRef

LLVM change 5fbd1a3 modified this
function to want std::string instead of StringRef, which is easily done.

Author: durin42

compiler/rustc_llvm/llvm-wrapper/CoverageMappingWrapper.cpp

@@ -23,10 +23,17 @@ extern "C" void LLVMRustCoverageWriteFilenamesSectionToBuffer(
     const char* const Filenames[],
     size_t FilenamesLen,
     RustStringRef BufferOut) {
+#if LLVM_VERSION_GE(12,0)
+  SmallVector<std::string,32> FilenameRefs;
+  for (size_t i = 0; i < FilenamesLen; i++) {
+    FilenameRefs.push_back(std::string(Filenames[i]));
+  }
+#else
   SmallVector<StringRef,32> FilenameRefs;
   for (size_t i = 0; i < FilenamesLen; i++) {
     FilenameRefs.push_back(StringRef(Filenames[i]));
   }
+#endif
   auto FilenamesWriter = coverage::CoverageFilenamesSectionWriter(
     makeArrayRef(FilenameRefs));
   RawRustStringOstream OS(BufferOut);