[analyzer] Don't crash if a block's signature just has the return type.

[analyzer] Don't crash if a block's signature just has the return type.

It is okay to declare a block without an argument list: ^ {} or ^void {}.
In these cases, the BlockDecl's signature-as-written will just contain
the return type, rather than the entire function type. It is unclear if
this is intentional, but the analyzer shouldn't crash because of it.

<rdar://problem/14018351>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182948 91177308-0d34-0410-b5e6-96231b3b80d8