[Coroutines] Clear FirstVLALoc in time

Unlike other *Loc member in FunctionScopeInfo, we didn't clear
FirstVLALoc in 'FunctionScopeInfo::Clear()'. Then it will be
problematic for the following case:

```
void bar(int n) {
  int array[n];
  return;
}

coroutine foo(int n) {
  co_return;
}
```

When we parse `foo`, the FirstVLALoc is still valid, then the compiler
will report `vla in coroutine` error in bar, which is super odd. After
this patch, we can fix this.

Author: ChuanqiXu9

clang/lib/Sema/ScopeInfo.cpp

@@ -39,6 +39,7 @@ void FunctionScopeInfo::Clear() {
   FirstReturnLoc = SourceLocation();
   FirstCXXOrObjCTryLoc = SourceLocation();
   FirstSEHTryLoc = SourceLocation();
+  FirstVLALoc = SourceLocation();
   FoundImmediateEscalatingExpression = false;
 
   // Coroutine state

clang/test/SemaCXX/coroutine-vla.cpp

@@ -16,6 +16,12 @@ struct promise
     void unhandled_exception();
 };
 
+// Test that we won't report the error incorrectly.
+void bar(int n) {
+  int array[n];
+  return;
+}
+
 coroutine foo(int n) {
   int array[n]; // expected-error {{variable length arrays in a coroutine are not supported}}
   co_return;