[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.

GitOrigin-RevId: 5641b1dfddff847f7f3edc484537f9314c283225
diff --git a/src/demangle/Utility.h b/src/demangle/Utility.h
index 04e1936..846a5f0 100644
--- a/src/demangle/Utility.h
+++ b/src/demangle/Utility.h
@@ -52,7 +52,7 @@
     char *TempPtr = std::end(Temp);
 
     while (N) {
-      *--TempPtr = '0' + char(N % 10);
+      *--TempPtr = char('0' + N % 10);
       N /= 10;
     }