Add a default for LLVM_CXX_STD in HandleLLVMOptions

[lanza]

Summary

A recent patch changed LLVM to use an LLVM_CXX_STD variable for
defining the C++ standard to use and also removed the default unset
case from HandleLLVMOption's usage. This broke standalone builds since,
for example, lldb's LLDBStandalone.cmake called HandleLLVMOptions.cmake
without this variable set.

Define a default value of c++11 for this value.

[tstellar]

cmake/modules/HandleLLVMOptions.cmake

440–442

I think all the default setting code from llvm/CMakeLists.txt should be moved to this file.

[tstellar]

Sorry didn't mean to accept the patch with my last comment.

[bogner]

This shouldn't be necessary any more after r357976. Sorry for the breakage!

cmake/modules/HandleLLVMOptions.cmake

if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT LLVM_ENABLE_LTO)
     endif()
   endif()
 
 elseif( LLVM_COMPILER_IS_GCC_COMPATIBLE )
   append_if(LLVM_ENABLE_WERROR "-Werror" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   append_if(LLVM_ENABLE_WERROR "-Wno-error" CMAKE_REQUIRED_FLAGS)
   add_flag_if_supported("-Werror=date-time" WERROR_DATE_TIME)
   add_flag_if_supported("-Werror=unguarded-availability-new" WERROR_UNGUARDED_AVAILABILITY_NEW)
+  if (NOT LLVM_CXX_STD)
+    set(LLVM_CXX_STD "c++11")
+  endif()
   check_cxx_compiler_flag("-std=${LLVM_CXX_STD}" CXX_SUPPORTS_CXX_STD)
   if (CXX_SUPPORTS_CXX_STD)
    if (CYGWIN OR MINGW)
       # MinGW and Cygwin are a bit stricter and lack things like
       # 'strdup', 'stricmp', etc in c++11 mode.
       string(REPLACE "c++" "gnu++" "${LLVM_CXX_STD}" gnu_LLVM_CXX_STD)
       append("-std=${gnu_LLVM_CXX_STD}" CMAKE_CXX_FLAGS)
     else()