[CMake] Remove CLANG_DEFAULT_STD_C/CLANG_DEFAULT_STD_CXX

When Clang is used as a cross compiler, it should respect the target
platform default. Defaulting to a configure-time CLANG_DEFAULT_STD_C/CLANG_DEFAULT_STD_CXX
does not make much sense.

When Clang is used as a single-platform compiler, we should use Clang
configuration file (https://discourse.llvm.org/t/configuration-files/42529
https://clang.llvm.org/docs/UsersManual.html#configuration-files).

The Gentoo needs from D34365 has been satisfied by a configuration file.

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D133375