VS: Add support for C++ module internal partitions in VS 17.6 and newer
VS 17.6 now implements ScanSourceforModuleDependencies
using the same
cl /scanDependencies
scanner that our Ninja generator uses. It can
distinguish module internal partitions from module interface units based
on their content. Switch from CompileAsCppModule
to CompileAsCpp
for CXX_MODULES
sources so that MSBuild can scan and classify them.