[IR] Make paramHasAttr to use arg indices instead of attr indices

[IR] Make paramHasAttr to use arg indices instead of attr indices

This avoids the confusing 'CS.paramHasAttr(ArgNo + 1, Foo)' pattern.

Previously we were testing return value attributes with index 0, so I
introduced hasReturnAttr() for that use case.

llvm-svn: 300367