Improve our handling of tag decls in function prototypes

r289225 broke AST invariants by reparenting enumerators into function
decl contexts. This improves things by only reparenting TagDecls while
also attempting to preserve the lexical declcontext chain. The
interesting example here is:
  int f(struct S { enum E { a = 1 } b; } c);

The semantic contexts of E and S should be f, and the lexical context of
S should be f and the lexical context of E should be S. We didn't do
that with r289225, but now we should.

This change should also improve our behavior on this example:
  void f() {
    extern void ext(struct S { } o);
    // S injected here
  }

Before r289225 we would only remove 'S' from the surrounding tag
injection context if it was the TU, but now we properly reparent S from
f to ext.

Fixes PR31366

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289678 91177308-0d34-0410-b5e6-96231b3b80d8