[clang] NFCI: Split `HeaderSearch::findAllModulesForHeader()`

This mimics the `ModuleMap` API and enables D151854, where the `AllowCreation = true` function needs `FileEntryRef` but `AllowCreation = false` functions is happy with plain `FileEntry`. No functional change intended.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D151853