Simplify the constructor to CodeGenABITypes.

The CodeGenOptions are not used for ABI type selection, so we will just
create one with the default constructor (there is a FloatABI option in
CodeGenOptions that is passed on to LLVM, but not used in Clang for LLVM
IR type generation).

We can use the DiagnosticsEngine on the ASTContext rather than making a
client pass one in explicitly.
Add an AdjustedType sugar node for adjusting calling conventions

Summary:
In general, this type node can be used to represent any type adjustment
that occurs implicitly without losing type sugar.  The immediate use of
this is to adjust the calling conventions of member function pointer
types without breaking template instantiation.

Fixes PR17996.

Reviewers: rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D2332