OSV-2020-1644 Segv on unknown address in clang::Sema::PushDeclContext

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=25059 Crash type: Segv on unknown address Crash state: clang::Sema::PushDeclContext clang::Sema::ActOnStartDelayedMemberDeclarations clang::Parser::ParseLexedPragmas...