[IR] Make getParamAttributes take argument numbers, not ArgNo+1

Add hasParamAttribute() and use it instead of hasAttribute(ArgNo+1,
Kind) everywhere.

The fact that the AttributeList index for an argument is ArgNo+1 should
be a hidden implementation detail.

NFC

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300272 91177308-0d34-0410-b5e6-96231b3b80d8