[analyzer] BlockCall shouldn't really be an AnyFunctionCall.

Per discussion with Anna a /long/ time ago, it was way too easy to misuse
BlockCall: because it inherited from AnyFunctionCall (through SimpleCall),
getDecl() was constrained to return a FunctionDecl, and you had to call
getBlockDecl() instead. This goes against the whole point of CallEvent
(to abstract over different ways to invoke bodies of code).

Now, BlockCall just inherits directly from CallEvent. There's a bit of
duplication in getting things out of the origin expression (which is still
known to be a CallExpr), but nothing significant.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199321 91177308-0d34-0410-b5e6-96231b3b80d8