Pull in r320755 from upstream clang trunk (by me):

Pull in r320755 from upstream clang trunk (by me):

  Don't trigger -Wuser-defined-literals for system headers

  Summary:
  In D41064, I proposed adding `#pragma clang diagnostic ignored
  "-Wuser-defined-literals"` to some of libc++'s headers, since these
  warnings are now triggered by clang's new `-std=gnu++14` default:

  $ cat test.cpp
  #include <string>

  $ clang -std=c++14 -Wsystem-headers -Wall -Wextra -c test.cpp
  In file included from test.cpp:1:
  In file included from /usr/include/c++/v1/string:470:
  /usr/include/c++/v1/string_view:763:29: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
      basic_string_view<char> operator "" sv(const char *__str, size_t __len)
                              ^
  /usr/include/c++/v1/string_view:769:32: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
      basic_string_view<wchar_t> operator "" sv(const wchar_t *__str, size_t __len)
                                 ^
  /usr/include/c++/v1/string_view:775:33: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
      basic_string_view<char16_t> operator "" sv(const char16_t *__str, size_t __len)
                                  ^
  /usr/include/c++/v1/string_view:781:33: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
      basic_string_view<char32_t> operator "" sv(const char32_t *__str, size_t __len)
                                  ^
  In file included from test.cpp:1:
  /usr/include/c++/v1/string:4012:24: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
      basic_string<char> operator "" s( const char *__str, size_t __len )
                         ^
  /usr/include/c++/v1/string:4018:27: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
      basic_string<wchar_t> operator "" s( const wchar_t *__str, size_t __len )
                            ^
  /usr/include/c++/v1/string:4024:28: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
      basic_string<char16_t> operator "" s( const char16_t *__str, size_t __len )
                             ^
  /usr/include/c++/v1/string:4030:28: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
      basic_string<char32_t> operator "" s( const char32_t *__str, size_t __len )
                             ^
  8 warnings generated.

  Both @aaron.ballman and @mclow.lists felt that adding this workaround
  to the libc++ headers was the wrong way, and it should be fixed in
  clang instead.

  Here is a proposal to do just that.  I verified that this suppresses
  the warning, even when -Wsystem-headers is used, and that the warning
  is still emitted for a declaration outside of system headers.

  Reviewers: aaron.ballman, mclow.lists, rsmith

  Reviewed By: aaron.ballman

  Subscribers: mclow.lists, aaron.ballman, andrew, emaste, cfe-commits

  Differential Revision: https://reviews.llvm.org/D41080

This will allow to compile some of the libc++ headers in C++14 mode
(which is the default for gcc 6 and higher, and will be the default for
clang 6.0.0 and higher), with -Wsystem-headers and -Werror enabled.

Reported by:	andrew
MFC after:	3 days