Fix several warnings exposed in MSVS 2015.

Fix several warnings exposed in MSVS 2015.

diagnostic.cpp:
- unreachable code

operand.cpp
- conversion between int and uint32_t
- unreachable code

hex_float.h:
- conversion from 'const int' to 'unsigned int'
- unreachable code

validate_id.cpp
- forcing value to bool 'true' or 'false'

validate_types.cpp:
- forcing value to bool 'true' or 'false'