[BuilderTransform] Make sure that synthesized if elements have correct locations

Placeholder variable that represents result of `if` should be placed
at the beginning of the statement, same goes for `Optional(.some(...))`
that wraps the expression in "then" branch.

Resolves: #62848

Author: xedin

lib/Sema/BuilderTransform.cpp

@@ -1275,7 +1275,7 @@ class ResultBuilderTransform
         // The operand should have optional type if we had optional results,
         // so we just need to call `buildIf` now, since we're at the top level.
         if (isOptional && isTopLevel(anchor)) {
-          builderCall = buildCallIfWanted(ifStmt->getEndLoc(),
+          builderCall = buildCallIfWanted(ifStmt->getThenStmt()->getStartLoc(),
                                           builder.getBuildOptionalId(),
                                           builderCall, /*argLabels=*/{});
         }
@@ -1304,7 +1304,7 @@ class ResultBuilderTransform
             /*argLabels=*/{}));
       }
 
-      auto *ifVarRef = builder.buildVarRef(ifVar.get(), ifStmt->getEndLoc());
+      auto *ifVarRef = builder.buildVarRef(ifVar.get(), ifStmt->getStartLoc());
       doBody.push_back(TypeJoinExpr::create(ctx, ifVarRef, buildEitherCalls));
     }
 

test/SourceKit/Sema/issue62848.swift

@@ -0,0 +1,23 @@
+// RUN: %sourcekitd-test -req=collect-type %s -- %s
+
+struct Rectangle {
+  init() {}
+}
+
+@resultBuilder public struct WiewBuilder {
+  public static func buildBlock<Content>(_ content: Content) -> Content {
+    return content
+  }
+  public static func buildIf<Content>(_ content: Content?) -> Content? {
+    return content
+  }
+}
+
+public struct AStack<Content> {
+  init(@WiewBuilder content: () -> Content) {}
+}
+
+func foo() {
+  AStack {
+    if true {
+      Rectangle()