If these are not detected then we cannot support the IAR compiler.
Fail early with an explicit message instead of silently proceeding
and getting strange errors.
Issue: #18333
Make the implementation for this compiler more complete.
IAR has multiple C++ modes, historically they were reduced c++ versions
for embedded that gradually improved to the full standard (which can be
reduced again by e.g. disabling rtti and exceptions). The new
implementation picks the best available, but the c++ mode can also be
overridden by defining `CMAKE_IAR_CXX_FLAG`.
Add C/C++ standard flags so that all modes up to and including the last
supported standard are defined.
Fixes: #16826
Teach the Makefile and Ninja generators to substitute for an <INCLUDES>
placeholder instead of putting -I in <FLAGS>. Update our values for
CMAKE_<LANG>_COMPILE_OBJECT,
CMAKE_<LANG>_CREATE_ASSEMBLY_SOURCE, and
CMAKE_<LANG>_CREATE_PREPROCESSED_SOURCE
to place <INCLUDES> just before <FLAGS>.
