Remove unnecessary StringRef convesion in llvm-config

[Ericson2314]

Summary

We have a string litteral (via CPP) used to construct StringRef, which
is used to construct a SmallString. Just construct the latter
directly.

[craig.topper]

I think this still ends up doing an implicit conversion to StringRef first since there is no SmallString constructor from const char *. But I guess shorter code is better.

LGTM

llvm/tools/llvm-config/llvm-config.cpp

if (IsInDevelopmentTree) {
     }
 
     // We need to include files from both the source and object trees.
     ActiveIncludeOption =
         ("-I" + ActiveIncludeDir + " " + "-I" + ActiveObjRoot + "/include");
   } else {
     ActivePrefix = CurrentExecPrefix;
     ActiveIncludeDir = ActivePrefix + "/include";
-    SmallString<256> path(StringRef(LLVM_TOOLS_INSTALL_DIR));
+    SmallString<256> path(LLVM_TOOLS_INSTALL_DIR);
     sys::fs::make_absolute(ActivePrefix, path);
     ActiveBinDir = std::string(path.str());
     ActiveLibDir = ActivePrefix + "/lib" + LLVM_LIBDIR_SUFFIX;
     ActiveCMakeDir = ActiveLibDir + "/cmake/llvm";
     ActiveIncludeOption = "-I" + ActiveIncludeDir;
   }
 
   /// We only use `shared library` mode in cases where the static library form