Discourage APIs locale-dependent APIs like String.to{Lower,Upper}Case

[cushon]

Inspired by bazelbuild/bazel#17687

Java's String#to{Lower,Upper}Case() is locale-dependent, which can
lead to unexpected results in locales with special case mappings in the
ASCII range (e.g. in a Turkish locale, a capital ASCII I lowercases to
a non-ASCII variant of i).

We have an internal refaster template that suggests com.google.common.base.Ascii, so we'd get this for free from #649

[fmeum]

@cushon Given that #649 has an unclear future, would you accept contributions for this?

[cushon]

Yes, absolutely

[rickie]

In Error Prone Support we have the StringCaseLocaleUsage check (link) exactly for this purpose.

We made it an Error Prone check, instead of a Refaster template, to show both Locale.ROOT and Locale.getDefault() in the suggested fix. We don't use the option to have multiple after-templates per Refaster rule.

We are definitely open to contributing this rule upstream (possibly with some tweaks to the BugPattern for e.g. the order of suggested fixes?). Would that be an option?

(Look here to see some extra documentation on the check.)

[rickie]

Filed #3813.