[libc++] Mark a few more tests as unsupported on gcc-8/9.

[libc++] Mark a few more tests as unsupported on gcc-8/9.

This will fix remaining failures on gcc-9 buildbot: http://lab.llvm.org:8011/#/builders/101.
gcc-8 and gcc-9 do not support constexpr destructors nor constexpr allocation.

Fix gcc warnings: -Wconversion, -Wpragmas.