[CodeCompletion] Fix issue in which parts of a result builder were incorrectly skipped

`getLoc` does not necesarrily return the start location of the location (e.g. for `a.b().c()` it returns the location of `c` because that’s the location of the call). But we used the location from `getLoc` as the start location of the synthesized `buildExpression` call. In the added test case, this means that the `buildExpression` call only contained `everlay() {}` and not the code completion token. We thus infered that we could skip it the entire `MyStack {}.pnTabGesture {}.everlay() {}` call for code completion, which isn’t correct.

rdar://120798355

Author: ahoppen

Diff for: lib/Sema/BuilderTransform.cpp

@@ -227,8 +227,8 @@ class ResultBuilderTransform
                                    buildBlockArguments);
     }
     if (builder.supports(ctx.Id_buildExpression)) {
-      expr = builder.buildCall(expr->getLoc(), ctx.Id_buildExpression, {expr},
-                               {Identifier()});
+      expr = builder.buildCall(expr->getStartLoc(), ctx.Id_buildExpression,
+                               {expr}, {Identifier()});
     }
 
     if (isa<CodeCompletionExpr>(expr)) {

Diff for: validation-test/IDE/issues_fixed/rdar120798355.swift

@@ -0,0 +1,31 @@
+// RUN: %batch-code-completion
+
+func test() {
+  MyStack {
+    MyStack {
+    }
+    .pnTapGesture {
+      #^COMPLETE^#
+    }
+    .everlay() {
+    }
+  }
+}
+
+struct MyView {
+  func everlay(content: () -> Void) -> MyView { MyView() }
+}
+
+struct MyStack {
+  init(@WiewBuilder content: () -> MyView) {}
+  func pnTapGesture(perform action: () -> Void) -> MyView { MyView() }
+}
+
+@resultBuilder
+struct WiewBuilder {
+  static func buildExpression(_ content: MyView) -> MyView { content }
+  static func buildBlock(_ content: MyView) -> MyView { content }
+  static func buildBlock() -> MyView { MyView() }
+}
+
+// COMPLETE: Decl[FreeFunction]/CurrModule:      test()[#Void#]