analyzer: more uninit test coverage

In addition to other test coverage, this adds the examples from
  https://cwe.mitre.org/data/definitions/457.html


(aka "CWE-457: Use of Uninitialized Variable")

For reference, the output from -fanalyzer looks like this
(after stripping away the DejaGnu directives):

uninit-CWE-457-examples.c: In function 'example_2_bad_code':
uninit-CWE-457-examples.c:56:3: warning: use of uninitialized value 'bN' [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
   56 |   repaint(aN, bN); /* { dg-warning "use of uninitialized value 'bN'" } */
      |   ^~~~~~~~~~~~~~~
  'example_2_bad_code': events 1-4
    |
    |   34 |   int aN, bN;
    |      |           ^~
    |      |           |
    |      |           (1) region created on stack here
    |   35 |   switch (ctl) {
    |      |   ~~~~~~
    |      |   |
    |      |   (2) following 'default:' branch...
    |......
    |   51 |   default:
    |      |   ~~~~~~~
    |      |   |
    |      |   (3) ...to here
    |......
    |   56 |   repaint(aN, bN);
    |      |   ~~~~~~~~~~~~~~~
    |      |   |
    |      |   (4) use of uninitialized value 'bN' here
    |
uninit-CWE-457-examples.c: In function 'example_3_bad_code':
uninit-CWE-457-examples.c:95:3: warning: use of uninitialized value 'test_string' [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
   95 |   printf("%s", test_string);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~
  'example_3_bad_code': events 1-4
    |
    |   90 |   char *test_string;
    |      |         ^~~~~~~~~~~
    |      |         |
    |      |         (1) region created on stack here
    |   91 |   if (i != err_val)
    |      |      ~
    |      |      |
    |      |      (2) following 'false' branch (when 'i == err_val')...
    |......
    |   95 |   printf("%s", test_string);
    |      |   ~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |   |
    |      |   (3) ...to here
    |      |   (4) use of uninitialized value 'test_string' here
    |

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/uninit-1.c: Add test coverage for shifts,
	comparisons, +, -, *, /, and __builtin_strlen.
	* gcc.dg/analyzer/uninit-CWE-457-examples.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>