Always use expand_symtabs_matching in ada-lang.c

The previous patch fixed the immediate performance problem with Ada
name matching, by having a subset of matches call
expand_symtabs_matching rather than expand_matching_symbols.  However,
it seemed to me that expand_matching_symbols should not be needed at
all.

To achieve this, this patch changes ada_lookup_name_info::split_name
to use the decoded name, rather than the encoded name.  In order to
make this work correctly, a new decoded form is used: one that does
not decode operators (this is already done) and also does not decode
wide characters.  The latter change is done so that changes to the Ada
source charset don't affect the DWARF index.

With this in place, we can change ada-lang.c to always use
expand_symtabs_matching rather than expand_matching_symbols.

gdb/ada-lang.h

@@ -216,10 +216,18 @@ extern const char *ada_decode_symbol (const struct general_symbol_info *);
    the name does not appear to be GNAT-encoded, then the result
    depends on WRAP.  If WRAP is true (the default), then the result is
    simply wrapped in <...>.  If WRAP is false, then the empty string
-   will be returned.  Also, when OPERATORS is false, operator names
-   will not be decoded.  */
+   will be returned.
+
+   When OPERATORS is false, operator names will not be decoded.  By
+   default, they are decoded, e.g., 'Oadd' will be transformed to
+   '"+"'.
+
+   When WIDE is false, wide characters will be left as-is.  By
+   default, they converted from their hex encoding to the host
+   charset.  */
 extern std::string ada_decode (const char *name, bool wrap = true,
-			       bool operators = true);
+			       bool operators = true,
+			       bool wide = true);
 
 extern std::vector<struct block_symbol> ada_lookup_symbol_list
      (const char *, const struct block *, domain_enum);