Completely reimplement __builtin_offsetof, based on a patch by Roberto

Amadini.

This change introduces a new expression node type, OffsetOfExpr, that
describes __builtin_offsetof. Previously, __builtin_offsetof was
implemented using a unary operator whose subexpression involved
various synthesized array-subscript and member-reference expressions,
which was ugly and made it very hard to instantiate as a
template. OffsetOfExpr represents the AST more faithfully, with proper
type source information and a more compact representation.

OffsetOfExpr also has support for dependent __builtin_offsetof
expressions; it can be value-dependent, but will never be
type-dependent (like sizeof or alignof). This commit introduces
template instantiation for __builtin_offsetof as well.

There are two major caveats to this patch:

  1) CodeGen cannot handle the case where __builtin_offsetof is not a
  constant expression, so it produces an error. So, to avoid
  regressing in C, we retain the old UnaryOperator-based
  __builtin_offsetof implementation in C while using the shiny new
  OffsetOfExpr implementation in C++. The old implementation can go
  away once we have proper CodeGen support for this case, which we
  expect won't cause much trouble in C++.

  2) __builtin_offsetof doesn't work well with non-POD class types,
  particularly when the designated field is found within a base
  class. I will address this in a subsequent patch.

Fixes PR5880 and a bunch of assertions when building Boost.Python
tests. 

llvm-svn: 102542