v8: unnecessary call to icu::toUCharPtr requires ICU 58+

[srl295]

Split off from #19151

I found an issue in v8 which I worked around at srl295@384ab7d - introduced into v8 at v8/v8@99e8963 (and into node later) - there is a call to icu::toUCharPtr() passed as an input to a reinterpret_cast. This seems to unnecessarily tie the code to ICU 59+, and in any event ICU uses char16_t and not UChar as the type for C++ going forward. So toUCharPtr() is not something to call going forward. Background here

If this were fixed, ICU4C 57 ought to be supported by master at this point.

[srl295]

Might be worked around by including the backported icu::toUCharPtr() function. edit yes, that seems to work. Will open a PR.

[bnoordhuis]

From your description of the issue, it sounds like V8 is the right place to fix this?

[srl295]

@bnoordhuis I now think so, per #19658 (comment)

for v8, what is needed here is either (1) fixing the call sites to stop using the unneeded casts or (2) including the backported char16ptr.h header. Both of these are small changes.

[bnoordhuis]

I can look into (1) probably tomorrow.

[bnoordhuis]

https://chromium-review.googlesource.com/c/v8/v8/+/987953