Add offsetof support to expression evaluator.

Summary:
We currently don't support offsetof in the expression evaluator as it is implemented as a macro
(which then calls __builtin_offsetof) in stddef.h. The best solution would be to include that
header (or even better, import Clang's builtin module), but header-parsing and
(cross-platform) importing modules is not ready yet.

Until we get this working with modules I would say we add the macro to our existing macro list
as we already do with other macros from stddef.h/stdint.h. We should be able to drop all of them
once we can import the relevant modules by default.

rdar://26040641

Reviewers: shafik, davide

Reviewed By: davide

Subscribers: clayborg, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64917

llvm-svn: 366476