Linespec lexing and C++ operators

Linespec lexing and C++ operators

There's some lexing code in linespec that isn't handling C++ operators
correctly.  It's the usual confusion with operator< / operator<<, in
code that wants to skip past template parameters.

The linespec_lexer_lex_string change is necessary otherwise we get
this (with current master):

 (gdb) break 'operator<'
 unmatched quote

The need for the find_toplevel_char change was exposed by the use of
that function in the explicit location completer.  Without the fix,
that completer is not able to "see" past operator< symbols, without
quoting, like:

 (gdb) b -function operator<(int, int) -labe[TAB]    # nothing happens

gdb incorrectly thinks "-labe" is part of the "unclosed" template
parameter list started with "<".

gdb/ChangeLog:
2017-07-17  Pedro Alves  <palves@redhat.com>

	* linespec.c (linespec_lexer_lex_string, find_toplevel_char):
	Handle 'operator<' / 'operator<<'.