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/handle_function.cpp

@@ -40,6 +40,8 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::FunctionIntroducerId node_id)
     -> bool {
+  // The function is potentially generic.
+  StartGenericDecl(context);
   // Create an instruction block to hold the instructions created as part of the
   // function signature, such as parameter and return types.
   context.inst_block_stack().Push();
@@ -48,8 +50,6 @@ auto HandleParseNode(Context& context, Parse::FunctionIntroducerId node_id)
   // Optional modifiers and the name follow.
   context.decl_introducer_state_stack().Push<Lex::TokenKind::Fn>();
   context.decl_name_stack().PushScopeAndStartName();
-  // The function is potentially generic.
-  StartGenericDecl(context);
   return true;
 }