[clang][ExprConst] Check record base classes for valid structs (#132270)

In error cases, the base might be None.

Fixes #132257

Author: tbaederr

clang/lib/AST/ExprConstant.cpp

@@ -7385,8 +7385,13 @@ class APValueToBufferConverter {
       for (size_t I = 0, E = CXXRD->getNumBases(); I != E; ++I) {
         const CXXBaseSpecifier &BS = CXXRD->bases_begin()[I];
         CXXRecordDecl *BaseDecl = BS.getType()->getAsCXXRecordDecl();
+        const APValue &Base = Val.getStructBase(I);
 
-        if (!visitRecord(Val.getStructBase(I), BS.getType(),
+        // Can happen in error cases.
+        if (!Base.isStruct())
+          return false;
+
+        if (!visitRecord(Base, BS.getType(),
                          Layout.getBaseClassOffset(BaseDecl) + Offset))
           return false;
       }

clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp

@@ -527,3 +527,17 @@ namespace test_complex {
   constexpr double D = __builtin_bit_cast(double, test_float_complex);
   constexpr int M = __builtin_bit_cast(int, test_int_complex); // expected-error {{size of '__builtin_bit_cast' source type 'const _Complex unsigned int' does not match destination type 'int' (8 vs 4 bytes)}}
 }
+
+namespace InvalidBaseClass {
+  class F {
+  public:
+    char c;
+  };
+
+  class InBetween : public F{};
+  class E : public InBetween {
+  public:
+    constexpr E() :  F{3} {} // expected-error {{not a direct or virtual base}}
+  };
+  static_assert(__builtin_bit_cast(char, E()) == 0); // expected-error {{not an integral constant expression}}
+}