[TypeChecker] Implement result builder transform via AST modification

The transform changes closure body into a multi-statement closure
with all of the implicit result builder calls and type-checks it
like a regular closure.

There are a couple of result builder specific changes mentioned below,
otherwise the logic to generate constraints and apply solutions is
unchanged:

- Placeholder variable: A variable declaration that doesn't have a
  type deduced and infers it from its first use. If such a variable
  has an initializer, it would be type-checked during solution application.

- TypeJoinExpr - an implicit expression that refers to a "join" variable
  and a set of expressions that should all produce the same type that
  becomes a type of a "join" variable.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -4824,6 +4824,16 @@ class ConstraintSystem {
   LLVM_NODISCARD
   bool generateConstraints(ClosureExpr *closure);
 
+  /// Generate constraints for the body of the given function.
+  ///
+  /// \param fn The function or closure expression
+  /// \param body The body of the given function that should be
+  /// used for constraint generation.
+  ///
+  /// \returns \c true if constraint generation failed, \c false otherwise
+  LLVM_NODISCARD
+  bool generateConstraints(AnyFunctionRef fn, BraceStmt *body);
+
   /// Generate constraints for the given (unchecked) expression.
   ///
   /// \returns a possibly-sanitized expression, or null if an error occurred.