allow @src() outside of functions and allow @src().line at comptime

[pfgithub]

It seems like @src() is intentionally runtime-only for incremental compilation reasons so this might not be wanted (#12016 (comment)). I would expect that column should be runtime-only then for the same reason.

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I was trying to generate shader source code at comptime and ran into these when trying to generate #line directives.

This changes @src() to work when not in a function. SourceLocation.fn_name is set to an empty string, which could potentially be a footgun but it seems hard to run into that by accident.

This also makes @src().line available at comptime.

fn demo() void {
    @compileLog(@src());
}

comptime {
    _ = demo;
    @compileLog(@src());
}

//Compile Log Output:
@as(builtin.SourceLocation, .{.file = "…/a.zig\x00", .fn_name = "\x00", .line = 7, .column = 17})
@as(builtin.SourceLocation, .{.file = "…/a.zig\x00", .fn_name = "demo\x00", .line = 2, .column = 17})

[Vexu]

@src is implemented as desired #2029 (comment), if you want to change that go through a proposal.