Compiler arguments coming from CC environment variables or multi-element
CMAKE_<LANG>_COMPILER variables set by toolchain files were previously not
exposed in the file API. Among other possible problems, this caused clients
to determine wrong system include paths and built-in preprocessor macros by
calling the compiler without these important arguments.
This is fixed by adding an optional "commandFragment" attribute to the
compiler description in the `toolchains` object, containing these arguments
as a command line fragment. This is already the form in which they are
internally stored in the CMAKE_<LANG>_COMPILER_ARG1 variable, so all that is
required is adding this variable to the set of exported variables, besides
some logic to omit it if empty.
Issue: #22568
Projects can use the new command to request file API replies for the current
run. No query files are generated, the query is tracked internally. Replies are
created in the file system at generation time in the usual way.
Fixes: #24951
