[C++20] Remove u8 string literal prefixes.

[allevato]

In C++20, u8 literals create values of type char8_t[] instead of char[], and these can't be implicitly converted.

Plan: Try just removing them first to see if any of our CI environments complain with un-prefixed Unicode strings. If they do, we can try introducing a macro that expands to u8 for C++<20 and to nothing for C++≥20. In some hypothetical future when everything is built in C++20 mode, we would want to address the usages then by explicitly using char8_t where needed.

[allevato]

@swift-ci please test

[allevato]

@swift-ci Please test Linux platform

[allevato]

@swift-ci Please test Windows platform

[allevato]

@swift-ci Please test Windows platform

[allevato]

Looks like removing u8 entirely breaks Windows (note the mojibake where the tau should be in the SIL function signature):

C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\test\Serialization\generic_extension.swift:12:11: error: CHECK: expected string not found in input
// CHECK: sil @$sSa19generic_extension_1E6wobble{{[_0-9a-zA-Z]*}}F : $@convention(method) <Ï„_0_0> (@guaranteed Array<Ï„_0_0>) -> @out Optional<Ï„_0_0>
<stdin>:1:1: note: scanning from here
sil_stage canonical
^
<stdin>:56:17: note: possible intended match here
 %36 = function_ref @$sSa19generic_extension_1E6wobblexSgyF : $@convention(method) <?_0_0> (@guaranteed Array<?_0_0>) -> @out Optional<?_0_0> // user: %37
                ^

[allevato]

The macro magic was needed after all. This introduces a SWIFT_UTF8("...") macro that should be used in lieu of direct u8"..." literals.

[allevato]

@swift-ci please test