Fix a failing assertion with -Wcast-align

When there is unknown type in a struct in code compiled with
-Wcast-align, the compiler crashes due to
clang::ASTContext::getASTRecordLayout() failing an assert.

Added check that the RecordDecl is valid before calling
getASTRecordLayout().

clang/lib/Sema/SemaChecking.cpp

@@ -14477,7 +14477,8 @@ static getBaseAlignmentAndOffsetFromLValue(const Expr *E, ASTContext &Ctx) {
   case Stmt::MemberExprClass: {
     auto *ME = cast<MemberExpr>(E);
     auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl());
-    if (!FD || FD->getType()->isReferenceType())
+    if (!FD || FD->getType()->isReferenceType() ||
+        FD->getParent()->isInvalidDecl())
       break;
     Optional<std::pair<CharUnits, CharUnits>> P;
     if (ME->isArrow())

clang/test/Sema/warn-cast-align.c

@@ -67,3 +67,11 @@ unsigned int func5(void);
 FnTy test5(void) {
   return (FnTy)&func5;
 }
+
+void test6() {
+  struct {
+    int hello;
+    doesnotexist world; // expected-error {{unknown type name 'doesnotexist'}}
+  } foo;
+  void **repro = (void **)&foo.hello; // expected-warning {{cast from 'int *' to 'void **' increases required alignment from 4 to 8}}
+}