import std with MSVC uses the arguments of the targets

With the cpp_importstd option and the MSVC compiler, meson now builds the module with the arguments of the targets that matter to the compiler when it imports the module: the C runtime, the character set, the C++ standard, the exception and floating point options, the macros read by the standard library, and so on. This prevents reporting a C505 warning, which would become an error if using werror. Arguments that do not matter, such as warnings, optimization and include directories, are ignored. A module is built for each different set of arguments, and shared by the targets that have the same one, so the files in the build directory are named std.<id>.ifc.

The module is no longer built through a compiler cache, which cannot restore it.