Improve error messages for attributes in the wrong context.

verifyFunctionAttrs has a comment that the value V is printed in error messages. The recently added errors for attributes didn't print V. Make them print V.

Change the stringification of AttributeList. Firstly they started with 'PAL[' which stood for ParamAttrsList. Change that to 'AttributeList[' matching its current name AttributeList. Print out semantic meaning of the index instead of the raw index value (i.e. 'return', 'function' or 'arg(n)').

Differential revision: https://reviews.llvm.org/D101484