ICU-20601 Wrap public ICU API compound macros in do { } while.

ICU-20601 Wrap public ICU API compound macros in do { } while.

This is the normal way of making compound macros behave like statements
and allowing macro invocations that look like function calls to work
just like if they were function calls.

Before ICU 65, function-like, multi-statement ICU macros were just
defined as series of statements wrapped in { } blocks and the caller
could choose to either treat them as if they were actual functions and
end the invocation with a trailing ; creating an empty statement after
the block or else omit this trailing ; using the knowledge that the
macro would expand to { }.

To continue supporting code that depends on that old behaviour, on being
able to omit the trailing ; when invoking one of these macros, the new
surrounding do { } while is done through preprocessor defines that can
easily be overridden at compile time, when needed.