RFR: 8261921: ClassListParser::current should be used only by main thread [v3]

[Ioi Lam]

> During -Xshare:dump, ClassListParser::current() should be used only by the main thread, which has created the only ClassListParser instance. Accessing it from other threads could cause intermittent failures. We observed this only on certain hosts with -Xcomp.
> 
> The fix is to check for `ClassListParser::is_parsing_thread()` before calling `ClassListParser::current()`. After the fix, I can no longer reproduce the crash.
> 
> I also did some renaming and comment cleaning.

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  removed unnecessary parenthesis

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2619/files
  - new: https://git.openjdk.java.net/jdk/pull/2619/files/08dc542e..fd902f17

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2619&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2619&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2619.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2619/head:pull/2619

PR: https://git.openjdk.java.net/jdk/pull/2619