[PATCH] Avoid infinite loop with duplicate anonymous union fields

[Bogdan Harjoc]

(this patch is already uploaded to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86690 )

If a struct contains an anonymous union and both have a field with the
same name, detect_field_duplicates_hash() will replace one of them
with NULL. If compilation doesn't stop immediately, it may later call
lookup_field() on the union, which falsely assumes the union's
LANG_SPECIFIC array is sorted, and may loop indefinitely because of
this.

Reproduced on amd64 since gcc-5, on ubuntu-18.04 and gentoo.

The patch falls back to iterating via DECL_CHAIN if there was an error
earlier during compilation.

I ran the gcc testsuite with the result (the FAIL seems unrelated to the patch):

FAIL: gcc.dg/cpp/_Pragma3.c (test for excess errors)

                === gcc Summary ===

# of expected passes            135094
# of unexpected failures        1
# of expected failures          398
# of unsupported tests          2140
gcc-build/gcc/xgcc  version 8.0.1 20180424 (experimental) (GCC)
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