Pull in r292833 from upstream libc++ trunk (by Eric Fiselier):

Pull in r292833 from upstream libc++ trunk (by Eric Fiselier):

  Manually force the use of __decltype in C++03 with Clang 3.4.

  <string> uses `decltype` in a way incompatible with `__typeof__`.
  This is problematic when compiling <string> with Clang 3.4 because
  even though it provides `__decltype` libc++ still used `__typeof__`
  because clang 3.4 doesn't provide __is_identifier which libc++
  uses to detect __decltype.

  This patch manually detects Clang 3.4 and properly configures
  for it.

This allows the graphics/openshadinglanguage port to build with
lang/clang34.

PR:		216054