[msan][test] Disable UB cases in vararg_shadow.cpp (#204246)

This patch comments out the tests for char and float, which have
undefined behavior: `warning: second argument to 'va_arg' is of
promotable type 'char'/'float'; this va_arg has undefined behavior
because arguments will be promoted to 'int'/'double' [-Wvarargs]`.

The non-UB way to rewrite the tests is to use `va_arg()` with int and
double types (when the original variables are char and float), but there
are already test cases for int and double.

Note also that the float test makes an assumption that
`cast<float>(cast<double>(float))` will maintain the original shadow,
which MSan need not guarantee. In particular,
https://github.com/llvm/llvm-project/pull/204197 breaks that assumption
(which led to a buildbot breakage and revert).

GitOrigin-RevId: 56a48d712006ff7d9a09eef6a86b78d4b2eec3aa