RFR: 8286117: Remove unnecessary indirection and unused code in UL

[Johan Sjölén]

This PR cleans up some minor annoyances in the UL code.

Note that the change from `LogImpl` to `LogTagSet` is equivalent, we're essentially inlining the calls that `LogImpl` would have made. This simplifies understanding ("why do we need `LogImpl` here?" was my question, answer: We don't!)'

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Commit messages:
 - Use uniform initialization syntax in top level
 - Use LogTagSet directly in LogMessage
 - Use LogTagSetMapping in LogTargetImpl instead of LogImpl
 - Declare pointers to LogTagSet const
 - Fix comments in LogStream

Changes: https://git.openjdk.java.net/jdk/pull/8541/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8541&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286117
  Stats: 18 lines in 5 files changed: 0 ins; 3 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8541.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8541/head:pull/8541

PR: https://git.openjdk.java.net/jdk/pull/8541