Update #pragmas to make clang be quiet about historic practice

This adds a #pragma to disable -Wdeprecated-register* on newer
versions of clang. We could remove all use of the register keyword
instead, as modern compilers ignore it. But it's not harmful, and
for the time being I prefer not to do doing any reformatting or
changing the historic character of this code base.

The #pragmas are removed from src/lib/libast/include/ast.h, because
they're better placed in src/lib/libast/features/common which
generates ast_common.h which is included by everything.

* https://clang.llvm.org/docs/DiagnosticsReference.html#wdeprecated-register

src/lib/libast/port/lcgen.c

@@ -20,6 +20,7 @@
 *                   Phong Vo <kpv@research.att.com>                    *
 *                                                                      *
 ***********************************************************************/
+#pragma clang diagnostic ignored "-Wdeprecated-register"
 #pragma clang diagnostic ignored "-Wparentheses"
 /*
  * generate <lc.h> implementation tables from lc.tab