[BUG] Capture considered local variable #850

Title: Capture considered local variable.

Description:

This is the example in the commit message of commit 4bd0c04.
But with a compound block instead.

Minimal reproducer (https://cpp2.godbolt.org/z/YTjP69cxP):

main: () = {
    v: std::vector = ( 1, 2, 3, 4, 5 );

    //  Definite last use of v => move-capture v into f's closure
    f := :() -> forward _ = { return v$; };

    //  Now we can access the vector captured inside f()...
    f().push_back(6);
    for f() do(e) std::cout << e;       // prints 123456
}

cppfront main.cpp2
clang++18 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -I . main.cpp

Expected result: Same as with f := :() -> forward _ = v$;.

Actual result and error: main.cpp2(5,31): error: a 'forward' return type cannot return a local variable.

See also:

• Commit 4bd0c04.
• [BUG] forward of capture $ crashes compiler #446.