RFR: 8370250: Locale should mention the behavior for duplicate subtags [v3]

[Justin Lu]

> Please review this PR which is a first stab at clarifying the behavior of duplicate variants, extension singletons, and U extension keys and attributes for BCP47 subtags in the Locale specification. This is a follow up to https://bugs.openjdk.org/browse/JDK-8369739.
> 
> Changes are made under the BCP47 and U extension sections that define "well-formed" in the class description.
> Additionally, changes are made under the relevant Locale and Locale.Builder methods themselves.
> 
> Will update the CSR accordingly when the proposed wording changes are finalized.

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  Add 'subsequent' wording to setLanguageTag

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27909/files
  - new: https://git.openjdk.org/jdk/pull/27909/files/59f52b32..daa7a2e6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27909&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27909&range=01-02

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27909.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27909/head:pull/27909

PR: https://git.openjdk.org/jdk/pull/27909