Store the mangled name as a string_view

Store the mangled name as a string_view

This should be a bit faster (because we can compare the size first),
but it is also a dependency for the next patch.

(3.47% of gdb startup time is spent in eq_demangled_name_entry when
attaching to Chrome's content_shell binary)

gdb/ChangeLog:

2019-10-22  Christian Biesinger  <cbiesinger@google.com>

	* symtab.c (struct demangled_name_entry): Change type of mangled
	to gdb::string_view. Also adds a constructor that takes the
	mangled name.
	(hash_demangled_name_entry): Update.
	(eq_demangled_name_entry): Update.
	(free_demangled_name_entry): New function to call the destructor
	now that this is not a POD anymore.
	(create_demangled_names_hash): Pass free_demangled_name_entry to
	htab_create_alloc.
	(symbol_set_names): Update.

Change-Id: I24711ae2bcaa9e79ca89a6f8fda385d400419175