Finish semantic analysis for [[carries_dependency]] attribute.

This required plumbing through a new flag to determine whether a ParmVarDecl is
actually a parameter of a function declaration (as opposed to a function
typedef etc, where the attribute is prohibited). Weirdly, this attribute (just
like [[noreturn]]) cannot be applied to a function type, just to a function
declaration (and its parameters).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173726 91177308-0d34-0410-b5e6-96231b3b80d8