RFR: 8377658: G1: Cleanup g1ConcurrentMark.cpp for stricter warning flags [v2]

[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`
> 
> `sizeof_auto` is created so that we can reduce the amount of casting and thus make the code more type safe. The normal `sizeof` will return a `size_t` although the size of most stuff can be represented in a `uint8_t`. `sizeof_auto`  will return the size in an as small unsigned type as is possible. The result is that expressions that uses `sizeof_auto` can convert to most integral types, and no explicit narrowing cast will be needed.

Leo Korinth has updated the pull request incrementally with four additional commits since the last revision:

 - fix copyright
 - split sizeof_auto to other enhancement
 - change cast to checked type
 - fix checked cast, fix field variable to uint

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/29701/files
  - new: https://git.openjdk.org/jdk/pull/29701/files/53190ad2..77fdc84a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29701&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29701&range=00-01

  Stats: 58 lines in 6 files changed: 0 ins; 52 del; 6 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