Sema: Preserve attributes on parameters in instantiated function templates.

This was causing correctness issues for ARC and the static analyzer when a
function template has "consumed" Objective-C object parameters (i.e.
parameters that will be released by the function before returning).

The fix is threefold:
(1) Actually copy over the attributes from old ParmVarDecls to new ones.
(2) Have Sema::BuildFunctionType only work for building FunctionProtoTypes,
    which it was doing anyway. This allows us to pass an ExtProtoInfo
    instead of a plain ExtInfo and several flags.
(3) Drop param attributes as part of StripImplicitInstantiation, which is
    used when an implicit instantiation is followed by an explicit one.

<rdar://problem/12685622>