Cleanup patch:

Semantics of parameters named Index and Idx were inconsistent between
"include/llvm/IR/Attributes.h", "lib/IR/AttributeImpl.h" and
"lib/IR/Attributes.cpp": sometimes these were fixed 1-based indexes of IR
parameters (or AttributeSet::ReturnIndex for IR return values or
AttributeSet::FunctionIndex for IR functions), other times they were the
internal slot for storage in the underlying AttributeSetImpl. I renamed usage of
the former to "Index" and usage of the latter to "Slot" ("Slot" was already
being used consistently for the latter in a subset of cases)

Patch by Stephen Lin!

llvm-svn: 179791