Change TypeSystem::GetBitSize() to return an optional result.

This patch changes the behavior when printing C++ function references:
where we previously would get a <could not determine size>, there is
now a <no summary available>. It's not clear to me whether this is a
bug or an omission, but it's one step further than LLDB previously
got.

Differential Revision: https://reviews.llvm.org/D56798

llvm-svn: 351376