FYI: build broken with LLVM 10 since 25 oct 2019

[Fangrui Song]

On 2019-10-30, Johan Engelen wrote:
>Before you spend time on it, see:
>https://lists.llvm.org/pipermail/llvm-dev/2019-October/136380.html
>
>We can fix this on our end too [1], but I'm waiting for what LLVM will 
>do.
>
>-Johan
>
>[1] For example:
>if(NOT (LDC_LLVM_VER LESS 1000))
>    set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform 
>to")
>    set(CMAKE_CXX_STANDARD_REQUIRED YES)
>    set(CMAKE_CXX_EXTENSIONS NO)
>endif()
>

I don't think llvm-config should print -std=c++14, because users may
want -std=c++17, -std=c++2a or -std=gnu++2a. Providing a default
CMAKE_CXX_STANDARD value should be ok but overriding it is probably not
a good idea.