[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
scan-build: use the xcodebuild specified by the user.

This is important if the user has multiple Xcodes installed on their
system -- we use xcodebuild to do a version check, and therefore we need
to make sure we match the actual build command.

Reported by Howard Ling!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182498 91177308-0d34-0410-b5e6-96231b3b80d8