C++20 coroutines introduces a new operator with a mangling of 'aw'.
This patch adds that to libiberty's demangler.

libiberty/ChangeLog:

2020-01-18  Iain Sandoe  <iain@sandoe.co.uk>

	* cp-demangle.c (cplus_demangle_operators): Add the co_await
	operator.
	* testsuite/demangle-expected: Test co_await operator mangling.