[Sema] Avoid forming double braced single expression closures

For a single expression closure, just use the
expression as the body in the case where we're
coercing to Void, as the return is already
implied. This avoids crashing in
`ClosureExpr::getSingleExpressionBody` with a
double braced body.

Surprisingly it seems nothing is currently calling
`ClosureExpr::getSingleExpressionBody` after
type-checking, so no test case, but later commits
in this patch will exercise this case.

Author: hamishknight

lib/Sema/CSSyntacticElement.cpp

@@ -1768,6 +1768,14 @@ class SyntacticElementSolutionApplication
       // Evaluate the expression, then produce a return statement that
       // returns nothing.
       TypeChecker::checkIgnoredExpr(resultExpr);
+
+      // For a single expression closure, we can just preserve the result expr,
+      // and leave the return as implied. This avoids neededing to jump through
+      // nested brace statements to dig out the single expression in
+      // ClosureExpr::getSingleExpressionBody.
+      if (context.isSingleExpressionClosure(cs))
+        return resultExpr;
+
       auto &ctx = solution.getConstraintSystem().getASTContext();
       auto newReturnStmt =
           new (ctx) ReturnStmt(

test/PCMacro/nested_function.swift

@@ -47,7 +47,5 @@ foo()
 // CHECK-NEXT: [19:3-19:8] pc before
 // CHECK-NEXT: [14:5-14:6] pc before
 // CHECK-NEXT: [14:5-14:6] pc after
-// CHECK-NEXT: [14:5-14:6] pc before
-// CHECK-NEXT: [14:5-14:6] pc after
 // CHECK-NEXT: [19:3-19:8] pc after
 // CHECK-NEXT: [22:1-22:6] pc after