[v3] constexpr complex, chrono, memory

[Benjamin Kosnik]

Assorted constexpr additions. 

Jonathan had suggested constexpr ctors for future, and
shared_future. Thus added. Remaining: unique_lock, exception_ptr?

Added tests for constexpr constructors for non-literal classes. This is
similar to the tests in the g++ testsuite, but instead of looking for
static_initialization_and_destruction, look for named constexpr
contructor. As discussed with Jason in Batavia. There was some thought
of providing an attribute syntax for this, say something like
__attribute__((init_constant)) that would warn when the variable
wasn't constant initialized. I'm still in favor of that, (Have added
this syntax to PR45923) but will use this method as the best currently
available. 

tested x86_64/linux

-benjamin
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