Fix PR 4489, a crash in PCH loading that occurs when loading the name

Fix PR 4489, a crash in PCH loading that occurs when loading the name
of a top-level declaration loads another top-level declaration of the
same name whose type depends on the first declaration having been
completed. This commit breaks the circular dependency by delaying
loads of top-level declarations triggered by loading a name until we
are no longer recursively loading types or declarations.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@74847 91177308-0d34-0410-b5e6-96231b3b80d8