cxxmodules: support `ifcMap` for module mappers
The -ifcMap
flag for MSVC takes a TOML file describing modules. Further metadata may be specified in here for future enhancement (e.g., P3041R0 proposes implementing importing STL headers via the std
IFC file directly rather than via header unit compilation).
https://learn.microsoft.com/en-us/cpp/build/reference/ifc-map
Cc: @brad.king