When building GDB on Centos 7 (which has flex 2.5.37) and Clang, I get:

    $ make ada-exp.o
      YACC   ada-exp.c
      LEX    ada-lex.c
      CXX    ada-exp.o
    In file included from /home/smarchi/src/binutils-gdb/gdb/ada-exp.y:1179:
    <stdout>:1106:2: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
     1106 |         register yy_state_type yy_current_state;
          |         ^~~~~~~~

In ada-lex.l, we already use `DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER`,
which for Clang translates to ignoring `-Wdeprecated-register` [1].  I think
that was produced when compiling as C++11, but now that we always compile as
C++17, Clang produces a `-Wregister` error [2].

For GCC, `DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER` already translates to
ignoring `-Wregister`.  So, rename
`DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER` to `DIAGNOSTIC_IGNORE_REGISTER`
and ignore `-Wregister` for Clang too.

[1] https://releases.llvm.org/17.0.1/tools/clang/docs/DiagnosticsReference.html#wdeprecated-register
[2] https://releases.llvm.org/17.0.1/tools/clang/docs/DiagnosticsReference.html#wregister

include/ChangeLog:

	* diagnostics.h (DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER): Rename
	to...
	(DIAGNOSTIC_IGNORE_REGISTER): ... this.  Ignore `-Wregister`
	instead of `-Wdeprecated-register`.

Change-Id: I8a4a51c7222c68577fa22ecacdddfcba32d9dbc5