Force the PS4 clang ABI version to 6.

The PS4 requires clang ABI version 6 for compatibility reasons. This change forces this and if the user specifies a different version when the PS4 target is specified, the compiler emits a warning that the specified version is being ignored.

Reviewers: probinson

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D46767

llvm-svn: 332160