[analyzer] Workaround crash on encountering Class non-type template parameters

The Clang Static Analyzer will crash on this code:
```lang=C++
struct Box {
  int value;
};
template <Box V> int get() {
  return V.value;
}
template int get<Box{-1}>();
```
https://godbolt.org/z/5Yb1sMMMb

The problem is that we don't account for encountering `TemplateParamObjectDecl`s
within the `DeclRefExpr` handler in the `ExprEngine`.

IMO we should create a new memregion for representing such template
param objects, to model their language semantics.
Such as:
 - it should have global static storage
 - for two identical values, their addresses should be identical as well
http://eel.is/c%2B%2Bdraft/temp.param#8

I was thinking of introducing a `TemplateParamObjectRegion` under `DeclRegion`
for this purpose. It could have `TemplateParamObjectDecl` as a field.

The `TemplateParamObjectDecl::getValue()` returns `APValue`, which might
represent multiple levels of structures, unions and other goodies -
making the transformation from `APValue` to `SVal` a bit complicated.

That being said, for now, I think having `Unknowns` for such cases is
definitely an improvement to crashing, hence I'm proposing this patch.

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D135763

(cherry picked from commit b062ee7)

Author: steakhal

clang/lib/StaticAnalyzer/Core/ExprEngine.cpp

@@ -2839,6 +2839,12 @@ void ExprEngine::VisitCommonDeclRefExpr(const Expr *Ex, const NamedDecl *D,
     return;
   }
 
+  if (const auto *TPO = dyn_cast<TemplateParamObjectDecl>(D)) {
+    // FIXME: We should meaningfully implement this.
+    (void)TPO;
+    return;
+  }
+
   llvm_unreachable("Support for this Decl not implemented.");
 }
 

clang/test/Analysis/template-param-objects.cpp

@@ -0,0 +1,33 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,debug.ExprInspection \
+// RUN:    -analyzer-config eagerly-assume=false -std=c++20 -verify %s
+
+template <class T> void clang_analyzer_dump(T);
+void clang_analyzer_eval(bool);
+
+struct Box {
+  int value;
+};
+bool operator ==(Box lhs, Box rhs) {
+  return lhs.value == rhs.value;
+}
+template <Box V> void dumps() {
+  clang_analyzer_dump(V);        // expected-warning {{lazyCompoundVal}}
+  clang_analyzer_dump(&V);       // expected-warning {{Unknown}}
+  clang_analyzer_dump(V.value);  // expected-warning {{Unknown}} FIXME: It should be '6 S32b'.
+  clang_analyzer_dump(&V.value); // expected-warning {{Unknown}}
+}
+template void dumps<Box{6}>();
+
+// [temp.param].7.3.2:
+// "All such template parameters in the program of the same type with the
+// same value denote the same template parameter object."
+template <Box A1, Box A2, Box B1, Box B2> void stable_addresses() {
+  clang_analyzer_eval(&A1 == &A2); // expected-warning {{UNKNOWN}} FIXME: It should be TRUE.
+  clang_analyzer_eval(&B1 == &B2); // expected-warning {{UNKNOWN}} FIXME: It should be TRUE.
+  clang_analyzer_eval(&A1 == &B2); // expected-warning {{UNKNOWN}} FIXME: It should be FALSE.
+
+  clang_analyzer_eval(A1 == A2); // expected-warning {{UNKNOWN}} FIXME: It should be TRUE.
+  clang_analyzer_eval(B1 == B2); // expected-warning {{UNKNOWN}} FIXME: It should be TRUE.
+  clang_analyzer_eval(A1 == B2); // expected-warning {{UNKNOWN}} FIXME: It should be FALSE.
+}
+template void stable_addresses<Box{1}, Box{1}, Box{2}, Box{2}>();