Fix comparator bug in cooked index

Simon pointed out that the cooked index template-matching patch
introduced a failure in libstdc++ debug mode.  In particular, the new
code violates the assumption of std::lower_bound and std::upper_bound
that the range is sorted with respect to the comparison.

When I first debugged this, I thought the problem was unfixable as-is
and that a second layer of filtering would have to be done.  However,
on irc, Simon pointed out that it could perhaps be solved if the
comparison function were assured that one operand always came from the
index, with the other always being the search string.

This patch implements this idea.

First, a new mode is introduced: a sorting mode for
cooked_index_entry::compare.  In this mode, strings are compared
case-insensitively, but we're careful to always sort '<' before any
other printable character.  This way, two names like "func" and
"func<param>" will be sorted next to each other -- i.e., "func1" will
not be seen between them.  This is important when searching.

Second, the compare function is changed to work in a strcmp-like way.
This makes it easier to test and (IMO) understand.

Third, the compare function is modified so that in non-sorting modes,
the index entry is always the first argument.  This allows consistency
in compares.

I regression tested this in libstdc++ debug mode on x86-64 Fedora 36.
It fixes the crash that Simon saw.

This is v2.  I believe it addresses the review comments, except for
the 'enum class' change, as I mentioned in email on the list.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/dwarf2/cooked-index.h

@@ -143,16 +143,54 @@ struct cooked_index_entry : public allocate_on_obstack
      STORAGE.  */
   const char *full_name (struct obstack *storage) const;
 
-  /* Compare two strings, case-insensitively.  Return true if STRA is
-     less than STRB.  If one string has template parameters, but the
-     other does not, then they are considered to be equal; so for
-     example "t<x>" == "t<x>", "t<x>" < "t<y>", but "t" == "t<x>".  */
-  static bool compare (const char *stra, const char *strb, bool completing);
+  /* Comparison modes for the 'compare' function.  See the function
+     for a description.  */
+  enum comparison_mode
+  {
+    MATCH,
+    SORT,
+    COMPLETE,
+  };
+
+  /* Compare two strings, case-insensitively.  Return -1 if STRA is
+     less than STRB, 0 if they are equal, and 1 if STRA is greater.
+
+     When comparing, '<' is considered to be less than all other
+     printable characters.  This ensures that "t<x>" sorts before
+     "t1", which is necessary when looking up "t".  This '<' handling
+     is to ensure that certain C++ lookups work correctly.  It is
+     inexact, and applied regardless of the search language, but this
+     is ok because callers of this code do more precise filtering
+     according to their needs.  This is also why using a
+     case-insensitive comparison works even for languages that are
+     case sensitive.
+
+     MODE controls how the comparison proceeds.
+
+     MODE==SORT is used when sorting and the only special '<' handling
+     that it does is to ensure that '<' sorts before all other
+     printable characters.  This ensures that the resulting ordering
+     will be binary-searchable.
+
+     MODE==MATCH is used when searching for a symbol.  In this case,
+     STRB must always be the search name, and STRA must be the name in
+     the index that is under consideration.  In compare mode, early
+     termination of STRB may match STRA -- for example, "t<int>" and
+     "t" will be considered to be equal.  (However, if A=="t" and
+     B=="t<int>", then this will not consider them as equal.)
+
+     MODE==COMPLETE is used when searching for a symbol for
+     completion.  In this case, STRB must always be the search name,
+     and STRA must be the name in the index that is under
+     consideration.  In completion mode, early termination of STRB
+     always results in a match.  */
+  static int compare (const char *stra, const char *strb,
+		      comparison_mode mode);
 
   /* Compare two entries by canonical name.  */
   bool operator< (const cooked_index_entry &other) const
   {
-    return compare (canonical, other.canonical, false);
+    return compare (canonical, other.canonical, SORT) < 0;
   }
 
   /* The name as it appears in DWARF.  This always points into one of