Fix typo in docs/GettingStarted.rst.
[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>
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!