RFR: 8267870: Remove unnecessary char_converter during class loading

[Coleen Phillimore]

We create a String object from a Symbol, then swap the '/' for '.' characters, and create another String object.  Seems wasteful.  Removed java_lang_String::char_converted and made externalize_classname take a Symbol.

Tested with tier1-3 in mach5.

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Commit messages:
 - Remove unnecessary char_converter

Changes: https://git.openjdk.java.net/jdk/pull/4173/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4173&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267870
  Stats: 76 lines in 5 files changed: 1 ins; 66 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4173.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4173/head:pull/4173

PR: https://git.openjdk.java.net/jdk/pull/4173