Fix assertion in ClangASTContext

Summary:
llvm::APSInt(0) asserts because it creates an int with bit-width 0 and
not (as I thought) a value 0.

Theoretically it should be sufficient to change this to APSInt(1), as
the intention there was that the value of the first argument should be
ignored if the type is invalid, but that would look dodgy.

Instead, I use llvm::Optional to denote an invalid value and use a
special struct instead of a std::pair, to reduce typing and increase
clarity.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D40615

llvm-svn: 319414