[modules] Delay calling DeclMustBeEmitted until it's safe.

This patch implements the suggestion in D29753 that calling DeclMustBeEmitted in
the middle of deserialization should be avoided and that the actual check should
be deferred until it's safe to do so.

This patch fixes a crash when accessing the invalid redecl chains while trying
to evaluate the value of a const VarDecl that contains a function call.

Patch by Raphael Isemann (D30793)!

llvm-svn: 300110