Fix a bug involving deferred decl emission and PCH

For various reasons, involving dllexport and class linkage compuations,
we have to wait until after the semicolon after a class declaration to
emit inline methods. These are "deferred" decls. Before this change,
finishing the tag decl would trigger us to deserialize some PCH so that
we could make a "pretty" IR-level type. Deserializing the PCH triggered
calls to HandleTopLevelDecl, which, when done, checked the deferred decl
list, and emitted some dllexported decls that weren't ready.

Avoid this re-entrancy. Deferred decls should not get emitted when a tag
is finished, they should only be emitted after a real top level decl in
the main file.

llvm-svn: 267186