[Build][NFC] Fixes for building on Windows with libc++

[courbet]

LGTM for llvm-exegesis.

llvm/include/llvm/Support/RWMutex.h

 };
 #endif
 
 /// SmartMutex - An R/W mutex with a compile time constant parameter that
 /// indicates whether this mutex should become a no-op when we're not
 /// running in multithreaded mode.
 template <bool mt_only> class SmartRWMutex {
   // shared_mutex (C++17) is more efficient than shared_timed_mutex (C++14)
-  // on Windows and always available on MSVC.
-#if defined(_MSC_VER) || __cplusplus > 201402L
+  // on Windows and always available on MSVC except with libc++.
+#if (defined(_MSC_VER) && !defined(_LIBCPP_VERSION)) || __cplusplus > 201402L
   std::shared_mutex impl;
 #else
 #if !defined(LLVM_USE_RW_MUTEX_IMPL)
   std::shared_timed_mutex impl;
 #else
   RWMutexImpl impl;
 #endif
 #endif

llvm/lib/Support/NativeFormatting.cpp

 //===- NativeFormatting.cpp - Low level formatting helpers -------*- C++-*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/NativeFormatting.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
 
+#if defined(_WIN32) && !defined(__MINGW32__)
+#include <float.h> // For _fpclass in llvm::write_double.
+#endif
+
 using namespace llvm;
 
 template<typename T, std::size_t N>
 static int format_to_buffer(T Value, char (&Buffer)[N]) {
   char *EndPtr = std::end(Buffer);
   char *CurPtr = EndPtr;
 
   do {

llvm/tools/llvm-exegesis/lib/X86/Target.cpp

 
 #include <memory>
 #include <string>
 #include <vector>
 #if defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
 #include <immintrin.h>
 #include <intrin.h>
 #endif
+#if defined(__x86_64__) && defined(_MSC_VER)
+#include <float.h> // For _clearfp in ~X86SavedState().
+#endif
 
 namespace llvm {
 namespace exegesis {
 
 static cl::OptionCategory
     BenchmarkOptions("llvm-exegesis benchmark x86-options");
 
 // If a positive value is specified, we are going to use the LBR in