Set CMAKE_<lang>_COMPILER_ID for VS generators

Currently the VS generators do not support Intel C/C++ .icproj files and
the MS tools do not include a Fortran compiler.  Therefore we can always
set the C and CXX compiler IDs to "MSVC" and the Fortran ID to "Intel".

This fixes a regression in support for the Intel Fortran compiler under
the VS plugin introduced by commit cd43636c (Modernize Intel compiler
info on Windows, 2010-12-16).  The commit moved the compiler information
into platform files that only load when the proper compiler id is set.
It worked for the NMake Makefiles generator but not for the VS IDE
generator because it did not set the compiler id.