CMake: simplified C++ codemodel interaction.

Now it uses ProjectPart::evaluateToolchain() to read toolchain info with
given compiler flags, so it saves old behavior and also reads C version
and C++ extensions.

Removed check that model doesn't need to be updated, since check didn't
account ProjectPart enums.

Change-Id: I6dbebeecdb162ec5b885f9f1846756b586c22b23
Reviewed-by: Daniel Teske <daniel.teske@digia.com>