OSV-2020-1408 Segv on unknown address in clang::Sema::LookupTemplateName

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=21152 Crash type: Segv on unknown address Crash state: clang::Sema::LookupTemplateName clang::Sema::isTemplateName clang::Parser::ParseOptionalCXXScopeSpecifier...