LWG Issue 2141: common type

[Marshall Clow]

Ref: http://cplusplus.github.io/LWG/lwg-defects.html#2141

Fix common_type to match the suggested implementation in the standard.
Add tests to make sure it (now) does what it should.

Also, a drive by fix on the tests for is_trivially_copyable. Clang recently fixed a bug in the intrinsic that libc++ uses for this type_trait, and this exposed an error in the test suite.

Note that most of the change in <type_traits> is from hoisting the definition of std::decay from the end of the file to somewhere closer to the front. 
That's just a movement of source code - there are no changes to decay.

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
        -- Yu Suzuki
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