RFR: 8312121: Fix -Wconversion warnings in tribool.hpp [v6]

[Coleen Phillimore]

> Assigning _value first, and then doing _value | 2 doesn't get -Wconversion warnings.  Also, reduced include file inclusion a little.
> Tested with tier1 on linux-x64-debug, windows-x64-debug, macos-aarch64-debug

Coleen Phillimore has updated the pull request incrementally with two additional commits since the last revision:

 - David suggestion
 - David suggestion

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/14892/files
  - new: https://git.openjdk.org/jdk/pull/14892/files/d380e8ef..288b47c2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14892&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14892&range=04-05

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14892.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14892/head:pull/14892

PR: https://git.openjdk.org/jdk/pull/14892