Reduce amount of work ODR hashing does.

When a FunctionProtoType is in the original type in a DecayedType, the decayed
type is a PointerType which points back the original FunctionProtoType.  The
visitor for ODRHashing will attempt to process both Type's, doing double work.
By chaining together multiple DecayedType's and FunctionProtoType's, this would
result in 2^N Type's visited only N DecayedType's and N FunctionProtoType's
exsit.  Another bug where VisitDecayedType and VisitAdjustedType did
redundant work doubled the work at each level, giving 4^N Type's visited.  This
patch removed the double work and detects when a FunctionProtoType decays to
itself to only check the Type once.  This lowers the exponential runtime to
linear runtime.  Fixes https://bugs.llvm.org/show_bug.cgi?id=41625

llvm-svn: 359960