[AST] Dont invalidate a ref-type var decl if it has no initializer.

[hokein]

Summary

This would allow more AST nodes being preserved for broken code, and
have a more consistent valid bit for ref-type var decl (currently, a
ref-type var decl with a broken initializer is valid).

Per https://reviews.llvm.org/D76831#1973053, the initializer of a variable
should play no part in its "invalid" bit.

[sammccall]

I'd be surprised if this *doesn't* break some fragile assumption somewhere, but it's a good change.

For the record, the setInvalid was added in e6565625f456123ad6eb33c84c8100d8f9411192. The other setInvalids from that patch:

• are for variables with incomplete/abstract types ==> these are correct I think
• are for init-sequence failures ==> these were removed by you in 9657385960350150b77ed652175b4c3801abd7fa and a forked case in 89d9912cbf45068770ac8c1e2ef97b74c3b662ab

So this feels like tying up a loose end consistent with prior work 👍

clang/lib/Sema/SemaDecl.cpp

if (Type->isIncompleteArrayType()) {
       return;
     }
 
     // Provide a specific diagnostic for uninitialized variable
     // definitions with reference type.
     if (Type->isReferenceType()) {
       Diag(Var->getLocation(), diag::err_reference_var_requires_init)
           << Var << SourceRange(Var->getLocation(), Var->getLocation());
-      Var->setInvalidDecl();
       return;
     }
 
     // Do not attempt to type-check the default initializer for a
     // variable with dependent type.
     if (Type->isDependentType())
       return;
 

clang/test/AST/ast-dump-invalid-initialized.cpp

 // RUN: not %clang_cc1 -triple x86_64-unknown-unknown -Wno-unused-value -fcxx-exceptions -std=gnu++17 -ast-dump %s | FileCheck -strict-whitespace %s
 
 struct A { A(int, int) {} };
 class ForwardDecl;
 
 void test() {
   // Verify the valid-bit of the VarDecl.
 
   // CHECK: `-VarDecl {{.*}} a1 'A'
   A a1;
   // CHECK: `-VarDecl {{.*}} a2 'const A'
   const A a2;
   // CHECK: `-VarDecl {{.*}} a3 'A'
   A a3 = garbage();
+  // CHECK: `-VarDecl {{.*}} a4 'const A &'
+  const A& a4;
 
 
-  // CHECK: `-VarDecl {{.*}} invalid b1 'const A &'
-  const A& b1;
   // CHECK: `-VarDecl {{.*}} invalid b2 'ForwardDecl'
   ForwardDecl b2;
   // CHECK: `-VarDecl {{.*}} invalid b3 'auto'
   auto b3 = garbage();
   // CHECK: `-VarDecl {{.*}} invalid b4 'auto'
   auto b4 = A(1);
   // CHECK: `-VarDecl {{.*}} invalid b5 'auto'
   auto b5 = A{1};
 }
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