Replace the linear search in find_pc_sect_line with a binary search.

This patch addresses slowness when setting breakpoints, especially in
heavily templatized code. Profiling showed that find_pc_sect_line in
symtab.c was the performance bottleneck.  The original logic performed a
linear search over ordered data. This patch uses a binary search, as
suggested by comments around the function.  There are no behavioural
changes, but gdb is now faster at setting breakpoints in template code.
Tested using on make check on an x86 target. The optimisation speeds up
the included template-breakpoints.py performance test by a factor of 7
on my machine.

ChangeLog:

2018-03-20  Stephen Roberts  <stephen.roberts@arm.com>

        * gdb/symtab.c (find_pc_sect_line): now uses binary search.

gdb/testsuite/

        * gdb.perf/template-breakpoints.cc: New file.
        * gdb.perf/template-breakpoints.exp: New file.
        * gdb.perf/template-breakpoints.py: New file.

gdb/testsuite/gdb.perf/template-breakpoints.py

@@ -0,0 +1,33 @@
+# Copyright (C) 2018 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from perftest import perftest
+
+class TemplateBreakpoints (perftest.TestCaseWithBasicMeasurements):
+    def __init__(self):
+        super (TemplateBreakpoints, self).__init__ ("template-breakpoints")
+
+    def warm_up(self):
+        for _ in range(0, 2):
+            gdb.Breakpoint("template-breakpoints.cc:38").delete()
+
+    def _do_test(self, bpcount):
+        for _ in range(1, bpcount):
+            gdb.Breakpoint("template-breakpoints.cc:38").delete()
+
+    def execute_test(self):
+        for bpcount in range(1, 10):
+            tfunc = lambda bound_bpcount=bpcount: self._do_test(bound_bpcount)
+            self.measure.measure(tfunc, bpcount)