pahole: Do --lang_exclude CU filtering earlier

With this, we can avoid warnings for unsupported DWARF tags like:

  die__process_function: tag not supported 0x2f (template_type_parameter)!

when processing object files generated from languages such as Rust, for
instance when building the Linux kernel with `CONFIG_RUST`, after
applying the next patch in this series.

As an added bonus, this should speed up processing of large objects with
filtered CUs, as their details will no longer be walked.

Committer testing:

Tested building a kernel with CONFIG_RUST, no warnings (that
template_type_param wouldn't appear anyway as I added initial support
for it, at least for creating some structs out of its DWARF info, so no
more warnings), but:

  ⬢[acme@toolbox linux]$ pahole -C '<kernel::str::RawFormatter as core::fmt::Write>::{vtable_type}' ../build/rust-kernel/rust/kernel.o
  die__process_class: tag not supported 0x33 (variant_part) at <14f43>!
  struct <kernel::str::RawFormatter as core::fmt::Write>::{vtable_type} {
          () *                       drop_in_place __attribute__((__aligned__(8))); /*     0     8 */
          usize                      size __attribute__((__aligned__(8))); /*     8     8 */
          usize                      align __attribute__((__aligned__(8))); /*    16     8 */
          () *                       __method3 __attribute__((__aligned__(8))); /*    24     8 */
          () *                       __method4 __attribute__((__aligned__(8))); /*    32     8 */
          () *                       __method5 __attribute__((__aligned__(8))); /*    40     8 */

          /* size: 48, cachelines: 1, members: 6 */
          /* forced alignments: 6 */
          /* last cacheline: 48 bytes */
  } __attribute__((__aligned__(8)));

  ⬢[acme@toolbox linux]$

That DW_TAG_variant_part warning would show up without this series, so
seems to be working as expected :-)

Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: rust-for-linux@vger.kernel.org
Link: https://lore.kernel.org/r/20240801185054.2518383-1-mmaurer@google.com
[ Split from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

pahole.c

@@ -3766,6 +3766,10 @@ int main(int argc, char *argv[])
 	memset(tab, ' ', sizeof(tab) - 1);
 
 	conf_load.steal = pahole_stealer;
+
+	if (languages.exclude)
+		conf_load.early_cu_filter = cu__filter;
+
 	conf_load.thread_exit = pahole_thread_exit;
 
 	if (conf_load.reproducible_build) {