[gdb/testsuite] Fix unrecognized debug output level 'statement-frontiers' message

[gdb/testsuite] Fix unrecognized debug output level 'statement-frontiers' message

When running testcase gdb.cp/step-and-next-inline.exp, I get:
...
 Running src/gdb/testsuite/gdb.cp/step-and-next-inline.exp ...
 gdb compile failed, g++: error: unrecognized debug output level \
   'statement-frontiers'
 gdb compile failed, g++: error: unrecognized debug output level \
   'statement-frontiers'

                 === gdb Summary ===

 # of untested testcases         2
...

Fix this by using a new gdb_caching_proc supports_statement_frontiers.

Tested on x86_64-linux, with gcc 7.5.0 (which does not support
-gstatement-frontiers) and with gcc 8.4.0 (which does support
-gstatement-frontiers).

gdb/testsuite/ChangeLog:

2020-03-14  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (supports_statement_frontiers): New proc.
	* gdb.cp/step-and-next-inline.exp: Use supports_statement_frontiers.