RFR: 8377658: G1: Cleanup g1ConcurrentMark.cpp for stricter warning flags [v3]

[Leo Korinth]

> Cleanup `g1ConcurrentMark.cpp` and included header files to remove implicit narrowing conversions.
> 
> For gcc: `-Wconversion -Wno-float-conversion`
> For clang: `-Wimplicit-int-conversion`

Leo Korinth has updated the pull request incrementally with one additional commit since the last revision:

  This will be fixed in JDK-8377995 instead

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/29701/files
  - new: https://git.openjdk.org/jdk/pull/29701/files/77fdc84a..a7af9b5e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29701&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29701&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29701.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29701/head:pull/29701

PR: https://git.openjdk.org/jdk/pull/29701