Fix C++ function params and return values printed in SemIR

[ivanaivanovska]

Fix pattern_block_id and call_params_id for C++ function decl import, to get the correct SemIR printout for the C++ functions i.e.

1. Fill in missing parameter info in the function decls:

// CHECK:STDOUT:   %foo.decl: %foo.type = fn_decl @foo [concrete = constants.%foo] {
// CHECK:STDOUT:     %a.patt: %pattern_type.2f8 = binding_pattern a [concrete]
// CHECK:STDOUT:     %a.param_patt: %pattern_type.2f8 = value_param_pattern %a.patt, call_param0 [concrete]
// CHECK:STDOUT:   } {
// CHECK:STDOUT:     %a.param: %i16 = value_param call_param0
// CHECK:STDOUT:     %.1: type = splice_block %i16 [concrete = constants.%i16] {
// CHECK:STDOUT:       %int_16: Core.IntLiteral = int_value 16 [concrete = constants.%int_16]
// CHECK:STDOUT:       %i16: type = class_type @Int, @Int(constants.%int_16) [concrete = constants.%i16]
// CHECK:STDOUT:     }
// CHECK:STDOUT:     %a: %i16 = bind_name a, %a.param
// CHECK:STDOUT:   }

2. Print the params and return values:

// CHECK:STDOUT: fn @foo_short() -> %i16;

// CHECK:STDOUT: fn @foo(%a.param: %i16);

Closes #5449

[ivanaivanovska]

CC: @geoffromer @bricknerb

[bricknerb]

Can we extract more logic from HandleAnyBindingPattern() so we can reuse it instead of doing it here, so we can more easily maintain this in case the logic changes?

[bricknerb]

Suggested change

	// Marks the start a region of insts, needed for the type
	// expression created later with the call of EndSubpatternAsExpr.
	// Mark the start of a region of insts, needed for the type expression
	// created later with the call of `EndSubpatternAsExpr()`.

[bricknerb]

Suggested change

	// TODO: Fix this once templates are supported.
	// TODO: Fix this once generics are supported.

[bricknerb]

If you can't rearrange the code so that Pop() is called in one place, consider using llvm::make_scope_exit() to make sure we call Pop().

[geoffromer]

I think you need to move this up, at least above GetReturnType, but preferably to the beginning of the function, because this block needs to hold all the non-pattern insts emitted as part of the function declaration. Right now the type insts for the return type are getting lost.

(Those insts don't really belong here, but we don't have a better place for them yet, so this code should be consistent with the native-Carbon code).