[modules] Set the lexical DC for dummy tag decls that refer to hidden

declarations that are made visible after the dummy is parsed and ODR verified

Prior to this commit the
"(getContainingDC(DC) == CurContext && "The next DeclContext should be lexically contained in the current one."),"
assertion failure was triggered during semantic analysis of the dummy
tag declaration that was declared in another tag declaration because its
lexical context did not point to the outer tag decl.

rdar://32292196

llvm-svn: 310706