Significantly rework the calculation of effective integer-expression ranges

for -Wsign-compare and -Wconversion, and use that coordinated logic to drive
both diagnostics.  The new logic works more transparently with implicit
conversions, conditional operators, etc., as well as bringing -Wconversion's
ability to deal with pseudo-closed operations (e.g. arithmetic on shorts) to
-Wsign-compare.

Fixes PRs 5887, 5937, 5938, and 5939.

llvm-svn: 92823