[analyzer] Don't crash if a block doesn't have a type signature.

Currently, blocks instantiated in templates lose their "signature as
written"; it's not clear if this is intentional. Change the analyzer's
use of BlockDecl::getSignatureAsWritten to check whether or not the
signature is actually there.

<rdar://problem/13954714>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182497 91177308-0d34-0410-b5e6-96231b3b80d8