[analyzer] Don't canonicalize the RecordDecl used in CXXBaseObjectRegion.

This Decl shouldn't be the canonical Decl; it should be the Decl used by
the CXXBaseSpecifier in the subclass. Unfortunately, that means continuing
to throw getCanonicalDecl() on all comparisons.

This fixes MemRegion::getAsOffset's use of ASTRecordLayout when redeclarations
are involved.