[libcxx] Prefer C++14 over C++11 when building libc++experimental.

[EricWF]

Summary

Currently libc++experimental builds with C++11. This patch changes that to C++14 when supported by the compiler. Although nothing currently requires C++14 the upcoming <experimental/memory_resource> implementation would benefit from it. [1]

Note that libc++.so continues to build with C++11 and is unaffected by this change.

[1] <experimental/memory_resource> provides global resources which must exist for the entire lifetime of the program. In order to ensure that a global resource can be used during program termination there destructors must never be invoked. The only way to do this, while also allowing "constant initialization", is to use a C++14 union.

libcxx/trunk/cmake/Modules/HandleLibcxxFlags.cmake

foreach(var ${ARGN})
     string(REPLACE "${var}" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
     string(REPLACE "${var}" "" CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS}")
     string(REPLACE "${var}" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
     string(REPLACE "${var}" "" CMAKE_SHARED_MODULE_FLAGS "${CMAKE_SHARED_MODULE_FLAGS}")
     remove_definitions(${var})
   endforeach()
 endmacro(remove_flags)
 
+macro(check_flag_supported flag)
+    mangle_name("${flag}" flagname)
+    check_cxx_compiler_flag("${flag}" "LIBCXX_SUPPORTS_${flagname}_FLAG")
+endmacro()
+
 # Add a macro definition if condition is true.
 macro(define_if condition def)
   if (${condition})
     add_definitions(${def})
   endif()
 endmacro()
 
 # Add a macro definition if condition is not true.

libcxx/trunk/lib/CMakeLists.txt

PROPERTIES
     VERSION       "${LIBCXX_ABI_VERSION}.0"
     SOVERSION     "${LIBCXX_ABI_VERSION}"
   )
 
 if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   file(GLOB LIBCXX_EXPERIMENTAL_SOURCES ../src/experimental/*.cpp)
   add_library(cxx_experimental STATIC ${LIBCXX_EXPERIMENTAL_SOURCES})
   target_link_libraries(cxx_experimental cxx)
+
+  set(experimental_flags "${LIBCXX_COMPILE_FLAGS}")
+  check_flag_supported(-std=c++14)
+  if (NOT MSVC AND LIBCXX_SUPPORTS_STD_EQ_CXX14_FLAG)
+    string(REPLACE "-std=c++11" "-std=c++14" experimental_flags "${LIBCXX_COMPILE_FLAGS}")
+  endif()
   set_target_properties(cxx_experimental
     PROPERTIES
-      COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
+      COMPILE_FLAGS "${experimental_flags}"
       OUTPUT_NAME   "c++experimental"
   )
 endif()
 
 # Generate a linker script inplace of a libc++.so symlink. Rerun this command
 # after cxx builds.
 if (LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
   # Get the name of the ABI library and handle the case where CXXABI_LIBNAME