This extends the existing discovery callback mechanism to report on functionsand methods. At this stage, we don't say much about them, in order to beconsistent with other discovery callbacks. Subsequent PRs will addextra callbacks to provide information especially about methods(virtualness, C++ visibility, etc.) Please request changes if you thinkthat sort of information should arrive in these callbacks.Because methods are a fundamentally C++ thing, this splits thecurrent ParseCallbacks test to cover both a .h and a .hpp header.Part ofgoogle/autocxx#124

No functional change - just deduplicating the logic which calls this callback,which will make it easier to make further changes in future.Part ofgoogle/autocxx#124

This adds more information to ParseCallbacks which indicates the location inthe original source code at which a given item was found.This has proven to be useful in downstream code generators in providingdiagnostics to explain why a given item can't be represented in Rust. (Thereare lots of reasons why this might not be the case - autocxx has around 100which can be found here -https://github.com/google/autocxx/blob/d85eac76c9b3089d0d86249e857ff0e8c36b988f/engine/src/conversion/convert_error.rs#L39- but irrespective of the specific reasons, it's useful to be able to point tothe original location when emitting diagnostics).Should we make this a new callback or include this information withinthe existing callback?Pros of making it a new callback:* No compatibility breakage.Pros of including it in this existing callback:* No need to specify and test a policy about whether such callbacks  always happen together, or may arrive individually* Easier for recipients (including bindgen's own test suite) to  keep track of the source code location received.* Because we add new items to the DiscoveryItem enum anyway,  we seem to have accepted it's OK to break compatibility in this  callback (for now at least).Therefore I'm adding it as a parameter to the existing callback. If it'sdeemed acceptable to break compatibility in this way, I will follow thesame thought process for some other changes too.Part ofgoogle/autocxx#124.

This makes two complementary improvements to the ParseCallbacks.The first is that Mods are now announced, as a new type ofDiscoveredItem. The second is that the parentage of each item isannounced. The parent of an item is often a mod (i.e. aC++ namespace) but not necessarily - it might be a struct withina struct, or similar.The reported information here is dependent on two pre-existingbindgen options:* whether to report C++ namespaces at all* whether to report inline namespaces conservatively.For that reason, the test suite gains two new tests.Part ofgoogle/autocxx#124

This change reports extra C++ information about items:* Whether methods are virtual or pure virtual or neither* Whether a method is a "special member", e.g. a move constructor* Whether a method is defaulted or deleted* C++ visibility (for structs, enums, unions and methods)It builds on top ofrust-lang#3145.A follow up PR should enhance the tests oncerust-lang#3139 is merged.Part ofgoogle/autocxx#124