SIL: hoist CFGState out of the function scope

[compnerd]

Visual Studio objects to the existing construct with:

lib\SIL\SILVerifier.cpp(4448): error C2065: 'Normal': undeclared identifier
lib\SIL\SILVerifier.cpp(4448): note: This diagnostic occurred in the compiler generated function '`anonymous-namespace'::SILVerifier::verifyFlowSensitiveRules::BBState::BBState(void)'
llvm\ADT\DenseMap.h(292): note: see reference to function template instantiation 'BucketT *llvm::DenseMapBase<llvm::DenseMap<swift::SILBasicBlock *,`anonymous-namespace'::SILVerifier::verifyFlowSensitiveRules::BBState,llvm::DenseMapInfo<KeyT>,BucketT>,KeyT,ValueT,KeyInfoT,BucketT>::InsertIntoBucket<const KeyT&,>(BucketT *,KeyArg)' being compiled
      with
      [
          BucketT=llvm::detail::DenseMapPair<swift::SILBasicBlock *,`anonymous-namespace'::SILVerifier::verifyFlowSensitiveRules::BBState>,
          KeyT=swift::SILBasicBlock *,
          ValueT=`anonymous-namespace'::SILVerifier::verifyFlowSensitiveRules::BBState,
          KeyInfoT=llvm::DenseMapInfo<swift::SILBasicBlock *>,
          KeyArg=swift::SILBasicBlock *const &
      ]

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[jrose-apple]

Using CFGState::Normal would be a more minimal fix. Does that work?

[compnerd]

I'm going to abandon this in favour of #19510