[NFC] Add an overload for getP1689ModuleDependencyFile without output parameters

See the comment of https://reviews.llvm.org/D153114. The current
signature will make the user to misunderstant that it will write files
to the filesystem, while it is not true. This patch adds an overload to
erase the concern. And the overload will be used in the support for
modules in clangd.