[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible loss...

[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible loss of data", part 2/7.

These tests for some guy's transparent operator functors were needlessly truncating their
double results to int. Preserving the doubleness makes compilers happier. I'm following
existing practice by adding an "// exact in binary" comment when the result isn't a whole number.
(The changes from 6 to 6.0 and so forth are stylistic, not critical.)

Fixes D27539.

llvm-svn: 289106