[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.

1 files changed, 4 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.cp/step-and-next-inline.exp b/gdb/testsuite/gdb.cp/step-and-next-inline.exp
index acec48b..3733fa7 100644
--- a/gdb/testsuite/gdb.cp/step-and-next-inline.exp
+++ b/gdb/testsuite/gdb.cp/step-and-next-inline.exp
@@ -15,6 +15,10 @@
 
 standard_testfile .cc
 
+if { ![supports_statement_frontiers] } {
+    return -1
+}
+
 # Compile the test source with USE_NEXT_INLINE_H defined (when
 # use_header is true), or not defined.
 proc do_test { use_header } {