Updated a comment about a clang bug.

tests/tcmp_ui.c

@@ -112,7 +112,9 @@ check_macros (void)
     {
       /* Failure in r13626 on x86_64 with the clang-9 1:9-1 Debian package,
          with any optimization level: c = 2 instead of 1
-         Bug report: https://bugs.llvm.org/show_bug.cgi?id=43557 */
+         Bug report: https://bugs.llvm.org/show_bug.cgi?id=43557
+         New URL: https://github.com/llvm/llvm-project/issues/42902
+         Apparently fixed in the clang-9 1:9.0.1-12 Debian package. */
       /* [2020-06-17]
          If one adds tcc support for macros using __builtin_constant_p
          in mpfr.h by testing __TINYC__, one also gets a failure.