[libc][benchmarks] Tidy uses of raw_string_ostream (NFC)

As specified in the docs,
1) raw_string_ostream is always unbuffered and
2) the underlying buffer may be used directly

( 65b1361 for further reference )

Avoid unneeded calls to raw_string_ostream::str(), to avoid excess indirection.

Author: JOE1994

libc/benchmarks/MemorySizeDistributions.cpp

@@ -197,7 +197,7 @@ getDistributionOrDie(ArrayRef<MemorySizeDistribution> Distributions,
          << "', available distributions:\n";
   for (const auto &MSD : Distributions)
     Stream << "'" << MSD.Name << "'\n";
-  report_fatal_error(Stream.str());
+  report_fatal_error(Message);
 }
 
 } // namespace libc_benchmarks

libc/benchmarks/automemcpy/unittests/CodeGenTest.cpp

@@ -50,7 +50,7 @@ TEST(Automemcpy, Codegen) {
   raw_string_ostream OutputStream(Output);
   Serialize(OutputStream, kDescriptors);
 
-  EXPECT_STREQ(OutputStream.str().c_str(),
+  EXPECT_STREQ(Output.c_str(),
                R"(// This file is auto-generated by libc/benchmarks/automemcpy.
 // Functions : 6