Fixed a bug where the parser-specific members of

persistent variables were staying around too long.
This caused the following problem:

- A persistent result variable is created for the
  result of an expression.  The pointer to the
  corresponding Decl is stored in the variable.

- The persistent variable is looked up during
  struct generation (correctly) using its Decl.

- Another expression defines a new result variable
  which happens to have a Decl in the same place
  as the original result variable.

- The persistent variable is looked up during
  struct generation using its Decl, but the old
  result variable appears first in the list and
  has the same Decl pointer.

The fix is to destroy parser-specific data when
it is no longer valid.

Also improved some logging as I diagnosed the
bug.

llvm-svn: 112540