RangePolicy construction fails if input bound types are not convertible to the policy's IndexType

[ldh4]

Reported from ECP-copa/Cabana#753 (comment).
 
Changes made in #6754 expects RangePolicy's input bound types to be convertible to the RangePolicy's IndexType.

However, the conversion safety check in RangePolicy actually requires a full roundtrip convertibility:

kokkos/core/src/Kokkos_ExecPolicy.hpp

Lines 271 to 272 in 506da18

	// check narrowing
	warn |= (static_cast<IndexType>(static_cast<member_type>(bound)) != bound);

and fails if only one way conversion is available.

Few options to mitigate this include:

• Adding constraints to the RangePolicy constructors (more conditions in enable_if)
• Making roundtrip convertibility a mandate (with a static_assert)
• Skipping the value preserving/narrowing check if only convertible one way (output a warning message about skipping the check)