[lld] Require C++14 in LLD standalone build

[int3]

Summary

This is what the Clang standalone build does too. And setting this
seems to be required to get the standalone build to work on my Mac.

[smeenai]

LGTM

[Ericson2314]

I wanted to check this is syntactically identical to Clang's, so we had a clear pattern going forward.

It is indeed! Thanks so much!

lld/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 # If we are not building as a part of LLVM, build LLD as an
 # standalone project, using LLVM as an external library:
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(lld)
   set(LLD_BUILT_STANDALONE TRUE)
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if(LLD_BUILT_STANDALONE)
+  set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
+  set(CMAKE_CXX_STANDARD_REQUIRED YES)
+  set(CMAKE_CXX_EXTENSIONS NO)
+
   set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
   # Rely on llvm-config.
   set(LLVM_CONFIG_OUTPUT)
   if(NOT LLVM_CONFIG)
    # back compat
    set(LLVM_CONFIG "${LLVM_CONFIG_PATH}")
   endif()