[Frontend] Avoid creating a temporary instance of std::string (NFC) (llvm#140326)

Since getLastArgValue returns StringRef, and the constructor of
SmallString accepts StringRef, we do not need to go through a
temporary instance of std::string.

Author: kazutakahirata

clang/lib/Frontend/CompilerInvocation.cpp

@@ -2055,8 +2055,7 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
   // The memory profile runtime appends the pid to make this name more unique.
   const char *MemProfileBasename = "memprof.profraw";
   if (Args.hasArg(OPT_fmemory_profile_EQ)) {
-    SmallString<128> Path(
-        std::string(Args.getLastArgValue(OPT_fmemory_profile_EQ)));
+    SmallString<128> Path(Args.getLastArgValue(OPT_fmemory_profile_EQ));
     llvm::sys::path::append(Path, MemProfileBasename);
     Opts.MemoryProfileOutput = std::string(Path);
   } else if (Args.hasArg(OPT_fmemory_profile))