Fix Ada crash with .debug_names

PR ada/25837 points out a crash in the gdb testsuite when .debug_names
is used.  You can reproduce like:

    runtest --target_board=cc-with-debug-names \
        gdb.ada/big_packed_array.exp

The bug was introduced by commit e0802d5 ("Avoid copying in
lookup_name_info").  The problem is that the return type of
language_lookup_name changed, but in a way that didn't cause existing
callers to trigger a compilation error.  Previously, it returned a
"const string &", but after it returned a "const char *".  This caused
a string to be created in dw2_expand_symtabs_matching_symbol, but one
that had too short of a lifetime; so eventually the matcher cache
would wind up with invalid data.

This patch fixes the problem by updating the callers to use the new
type.

Tested on x86-64 Fedora 30.

gdb/ChangeLog
2020-04-23  Tom Tromey  <[email protected]>

	PR ada/25837:
	* dwarf2/read.c (dw2_expand_symtabs_matching_symbol): Store a
	"const char *", not a "const std::string &".
	<name_and_matcher::operator==>: Update.
	* unittests/lookup_name_info-selftests.c: Change type of
	"result".

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2020-04-23  Tom Tromey  <[email protected]>
+
+	PR ada/25837:
+	* dwarf2/read.c (dw2_expand_symtabs_matching_symbol): Store a
+	"const char *", not a "const std::string &".
+	<name_and_matcher::operator==>: Update.
+	* unittests/lookup_name_info-selftests.c: Change type of
+	"result".
+
 2020-04-23  Tom Tromey  <[email protected]>
 
 	* inferior.h (iterate_over_inferiors): Don't declare.

gdb/dwarf2/read.c

@@ -3911,11 +3911,11 @@ dw2_expand_symtabs_matching_symbol
   struct name_and_matcher
   {
     symbol_name_matcher_ftype *matcher;
-    const std::string &name;
+    const char *name;
 
     bool operator== (const name_and_matcher &other) const
     {
-      return matcher == other.matcher && name == other.name;
+      return matcher == other.matcher && strcmp (name, other.name) == 0;
     }
   };
 

gdb/unittests/lookup_name_info-selftests.c

@@ -37,14 +37,14 @@ check_make_paramless (const char *file, int line,
 {
   lookup_name_info lookup_name (name, symbol_name_match_type::FULL,
 				completion_mode, true /* ignore_parameters */);
-  const std::string &result = lookup_name.language_lookup_name (lang);
+  const char *result = lookup_name.language_lookup_name (lang);
 
-  if (result != expected)
+  if (strcmp (result, expected) != 0)
     {
       error (_("%s:%d: make-paramless self-test failed: (completion=%d, lang=%d) "
 	       "\"%s\" -> \"%s\", expected \"%s\""),
 	     file, line, completion_mode, lang, name,
-	     result.c_str (), expected);
+	     result, expected);
     }
 }