fix: use tag type for calling convention

[ngynkvn]

Fix for 0.14.0-dev.1983+6bf52b050

std.builtin.CallingConvention was recently changed to union(enum(u8)), with some deprecated names as well. This swaps the field to be CallingConvention.Tag type instead and replaces the deprecated names with their equivalent

[ngynkvn]

I guess I shouldn't have expected this to be so simple 🙃

====== expected this output: =========
fn() align(4) callconv(.C) type␃

======== instead found this: =========
fn() align(4) callconv(.aarch64_aapcs_darwin) type␃

======================================
First difference occurs on line 1:
expected:
fn() align(4) callconv(.C) type
                        ^ ('\x43')
found:
fn() align(4) callconv(.aarch64_aapcs_darwin) type
                        ^ ('\x61')

I pushed up another change to adjust the test cases to fit the type change I made, but I don't think this will make them pass as per above. Feel free to close this if I'm chasing a red herring / understanding this incorrectly.