Attaching comments to redeclarations: fix wrong assumptions

The reason for the recent fallout for "attaching comments to any redeclaration"
change are two false assumptions:
(1) a RawComment is attached to a single decl (not true for 'typedef struct X *Y'
    where we want the comment to be attached to both X and Y);
(2) the whole redeclaration chain has only a single comment (obviously false, the
    user can put a separate comment for each redeclaration).

To fix (1) I revert the part of the recent change where a 'Decl*' member was
introduced to RawComment.  Now ASTContext has a separate DenseMap for mapping
'Decl*' to 'FullComment*'.

To fix (2) I just removed the test with this assumption.  We might not parse
every comment in redecl chain if we already parsed at least one.

llvm-svn: 161878