This patch addresses two cleanup issues:

1. Verify::VerifyParameterAttrs in "lib/IR/Verifier.cpp" and
   AttrBuilder::removeFunctionOnlyAttrs in "lib/IR/Attributes.cpp" (only called
   by Verify::VerifyFunctionAttrs) separately maintained a list of function-only
   attribute types. I've consolidated the logic into a new function used for
   both cases in "lib/IR/Verifier.cpp", so this logic is in one place (other
   than the AsmParser front-end)

2. Various functions in "lib/IR/Verifier.cpp" passed AttributeSet around by
   reference needlessly, as it's just a handle to an immutable pimpl body.

Patch by Stephen Lin!