Regressions on master at commit r12-6886 vs commit r12-6871 on Linux/x86_64

[H.J. Lu]

New failures:
FAIL: g++.dg/modules/xtreme-header-3_b.C -std=c++2b (internal compiler error: in add_mergeable_specialization, at cp/pt.cc:30458)
FAIL: g++.dg/modules/xtreme-header-3_b.C -std=c++2b (test for excess errors)

New passes:
FAIL: g++.dg/modules/xtreme-header-3_b.C -std=c++2a (internal compiler error: in hashtab_chk_error, at hash-table.cc:137)
FAIL: g++.dg/modules/xtreme-header-3_b.C -std=c++2a (test for excess errors)