Stop pushing a fake generic for the duration of check. (#5326)

This fake generic was used for two reasons:

- The declaration name stack assumes that each declaration name is
processed within a generic scope. This is important if the name might
have generic parameters, which are always parsed even for declarations
that disallow them in check.
- Out-of-line redeclarations of generic entities produce instructions
with symbolic constant values in non-generic scopes.

The former case is addressed by pushing a generic each time we start a
declaration name, even if we will reject generic parameters later. The
latter case is worked around for now by not building a symbolic constant
type or value for instructions that appear outside of any generic, and
will be addressed more completely by #5310 and follow-ups.

toolchain/check/generic_region_stack.cpp

@@ -11,6 +11,21 @@ auto GenericRegionStack::Push() -> void { dependent_insts_stack_.PushArray(); }
 auto GenericRegionStack::Pop() -> void { dependent_insts_stack_.PopArray(); }
 
 auto GenericRegionStack::AddDependentInst(DependentInst inst) -> void {
+  if (dependent_insts_stack_.empty()) {
+    // If we don't have a generic region here, leave the dependent instruction
+    // unattached. This happens for out-of-line redeclarations of members of
+    // dependent scopes:
+    //
+    //   class A(T:! type) {
+    //     fn F();
+    //   }
+    //   // Has generic type and constant value, but no generic region.
+    //   fn A(T:! type).F() {}
+    //
+    // TODO: Use a different instruction kind for out-of-line definitions and
+    // CHECK this doesn't happen.
+    return;
+  }
   dependent_insts_stack_.AppendToTop(inst);
 }