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.

llvm-svn: 196450