Set the variable added by commit a2112257 (Add infrastructure to use
language extensions without any standard, 2017-06-29) for IAR. This was
accidentally left out of commit d8e6cd9e (IAR: Improve support for IAR
ARM Compiler, 2017-06-15).
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
The dependency flags require recent versions of `iccarm` and `iccavr`.
The multi-rule dependency generated with `--dependencies=m` does not
work well with Ninja, so use `--dependencies=ns` instead.
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>.
