Cleaner implementation of MBEDTLS_IGNORE_RETURN
Cleaner implementation of MBEDTLS_IGNORE_RETURN

The previous implementation was misparsed in constructs like
`if (condition) MBEDTLS_IGNORE_RETURN(...); else ...;`.

Implement it as an expression, tested with GCC, Clang and MSVC.

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls