[libc++] [libc++abi] Mark a few tests as unsupported/xfail on gcc-7/8/9.

This should make the builder http://lab.llvm.org:8011/#/builders/101/ happy.
It uses gcc-9 and not Tip-Of-Trunk as its name indicates BTW.
GCC-10 passes all these tests.

Fix gcc warnings: -Wsign-compare, -Wparentheses, -Wpragmas.

Reviewed By: ldionne, #libc, #libc_abi

Differential Revision: https://reviews.llvm.org/D92099

libcxx/test/std/strings/basic.string/string.cons/string_view_deduction.fail.cpp

@@ -8,6 +8,7 @@
 
 // <string>
 // UNSUPPORTED: c++03, c++11, c++14
+// UNSUPPORTED: gcc-7
 // XFAIL: libcpp-no-deduction-guides
 
 // template<class InputIterator>