The no_unique_address attribute is not a reserved name until C++20, so
to use it in C++11/14/17 modes we should use the __no_unique_address_
form. We already use that form when using the attribute, but not in the
__has_cpp_attribute check.
libstdc++-v3/ChangeLog:
PR libstdc++/101055
* include/std/tuple: Use reserved form of attribute name.
* testsuite/17_intro/headers/c++2011/all_attributes.cc: Add
check for no_unique_address.
* testsuite/17_intro/headers/c++2014/all_attributes.cc:
Likewise.
* testsuite/17_intro/headers/c++2017/all_attributes.cc:
Likewise.
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