[OpenMP] libomp: Handle implicit conversion warnings

This patch partially prepares the runtime source code to be built with
-Wconversion, which should trigger warnings if any implicit conversions
can possibly change a value. For builds done with icc or gcc, all such
warnings are handled in this patch. clang gives a much longer list of
warnings, particularly for sign conversions, which the other compilers
don't report. The -Wconversion flag is commented into cmake files, but
I'm not going to turn it on. If someone thinks it is important, and wants
to fix all the clang warnings, they are welcome to.

Types of changes made here involve either improving the consistency of types
used so that no conversion is needed, or else performing careful explicit
conversions, when we're sure a problem won't arise.

Patch is a combination of changes by Terry Wilmarth and Johnny Peyton.

Differential Revision: https://reviews.llvm.org/D92942

GitOrigin-RevId: 6b316febb4388764789677f81f03aff373ec35b2
32 files changed