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.