[AST] Print correct tag decl for tag specifier

For example, given:

  void fn() {
    struct T *p0;
    struct T { int i; } *p1;
  }

-ast-print produced:

  void fn() {
    struct T { int i; } *p0;
    struct T { int i; } *p1;
  }

Compiling that fails with a redefinition error.

Given:

  void fn() {
    struct T *p0;
    struct __attribute__((deprecated)) T *p1;
  }

-ast-print dropped the attribute.

Details:

For a tag specifier (that is, struct/union/class/enum used as a type
specifier in a declaration) that was also a tag declaration (that is,
first occurrence of the tag) or tag redeclaration (that is, later
occurrence that specifies attributes or a member list), clang printed
the tag specifier as either (1) the full tag definition if one
existed, or (2) the first tag declaration otherwise.  Redefinition
errors were sometimes introduced, as in the first example above.  Even
when that was impossible because no member list was ever specified,
attributes were sometimes lost, thus changing semantics and
diagnostics, as in the second example above.

This patch fixes a major culprit for these problems.  It does so by
creating an ElaboratedType with a new OwnedDecl member wherever an
occurrence of a tag type is a (re)declaration of that tag type.
PrintingPolicy's IncludeTagDefinition used to trigger printing of the
member list, attributes, etc. for a tag specifier by using a tag
(re)declaration selected as described above.  Now, it triggers the
same thing except it uses the tag (re)declaration stored in the
OwnedDecl.  Of course, other tooling can now make use of the new
OwnedDecl as well.

Also, to be more faithful to the original source, this patch
suppresses printing of attributes inherited from previous
declarations.

Reviewed by: rsmith, aaron.ballman

Differential Revision: https://reviews.llvm.org/D45463

llvm-svn: 332281