[clang][deps] Make the C++ API more type-safe

Scanner's C++ API accepts a set of modular dependencies the client has already seen and for which it doesn't need the full details. This is currently a set of strings, which somewhat implies that it should contain the set of module names. However, scanner internally expects the values to be in the format "{hash}{name}". Besides not being documented, this is very unintuitive. This patch makes this expectation explicit by changing the type to set of `ModuleID`.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D156492