If a declaration is loaded, and then a module import adds a redeclaration, then
ensure that querying the first declaration for its most recent declaration
checks for redeclarations from the imported module.

This works as follows:
 * The 'most recent' pointer on a canonical declaration grows a pointer to the
   external AST source and a generation number (space- and time-optimized for
   the case where there is no external source).
 * Each time the 'most recent' pointer is queried, if it has an external source,
   we check whether it's up to date, and update it if not.
 * The ancillary data stored on the canonical declaration is allocated lazily
   to avoid filling it in for declarations that end up being non-canonical.
   We'll still perform a redundant (ASTContext) allocation if someone asks for
   the most recent declaration from a decl before setPreviousDecl is called,
   but such cases are probably all bugs, and are now easy to find.

Some finessing is still in order here -- in particular, we use a very general
mechanism for handling the DefinitionData pointer on CXXRecordData, and a more
targeted approach would be more compact.

Also, the MayHaveOutOfDateDef mechanism should now be expunged, since it was
addressing only a corner of the full problem space here. That's not covered
by this patch.

Early performance benchmarks show that this makes no measurable difference to
Clang performance without modules enabled (and fixes a major correctness issue
with modules enabled). I'll revert if a full performance comparison shows any
problems.