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

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.

Reviewed By: #lld-macho, MaskRay, Ericson2314, smeenai

Differential Revision: https://reviews.llvm.org/D120269

lld/CMakeLists.txt

@@ -11,6 +11,10 @@ endif()
 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.