[sil-verifier] Extract the flow sensitive verifier into its own file.

This has grown large enough and complex enough that it makes sense to go into
its own file.

This is a NFCI change. The only substantial changes is I added a small require
impl based on the one in SILVerifier and I eliminated the namespace
VerifyFlowSensitiveRulesDetails.

Author: gottesmm

lib/SIL/Verifier/CMakeLists.txt

@@ -1,5 +1,6 @@
 target_sources(swiftSIL PRIVATE
   DebugInfoVerifier.cpp
+  FlowSensitiveVerifier.cpp
   LinearLifetimeChecker.cpp
   MemoryLifetimeVerifier.cpp
   GuaranteedPhiVerifier.cpp