[sema] Wire up VarDecl parent pointers for case stmt related Var Decls

This is in preparation for fixing issues around SILGenPattern fallthrough
emission and bad rename/edit all in scope of case stmt var decls. Specifically,
I am going to ensure that we can get from any VarDecl in the following to any
other VarDecl:

switch x {
case .a(let v1, let v2), .b(let v1, let v2):
  ...
  fallthrough
case .c(let v1, let v2), .d(let v1, let v2):
  ...
}

This will be done by:

1. Pointing the var decls in .d at the corresponding var decls in .c.
2. Pointing the var decls in .c at the corresponding var decls in .b.
3. Pointing the var decls in .b at the corresponding var decls in .a.
4. Pointing the var decls in .a at the case stmt. Recognizing that we are asking
for the next VarDecl, but have a case stmt, we check if we have a fallthrough
case stmt. If so, follow down the fallthrough case stmts until you find a
fallthrough case stmt that doesn't fallthrough itself and then return the
corresponding var decl in the last case label item in that var decl (in the
above .d).

In a subsequent commit I am going to add case body var decls. The only change as
a result of that is that I will insert them into the VarDecl double linked list
after the last case var decl of each case stmt.

lib/Sema/MiscDiagnostics.cpp

@@ -2623,7 +2623,7 @@ VarDeclUsageChecker::~VarDeclUsageChecker() {
       }
       else {
         bool suggestLet = true;
-        if (auto *stmt = var->getParentPatternStmt()) {
+        if (auto *stmt = var->getRecursiveParentPatternStmt()) {
           // Don't try to suggest 'var' -> 'let' conversion
           // in case of 'for' loop because it's an implicitly
           // immutable context.