OSV-2020-1485 Segv on unknown address in clang::Sema::ActOnStartOfFunctionDef

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19623 Crash type: Segv on unknown address Crash state: clang::Sema::ActOnStartOfFunctionDef clang::Parser::ParseFunctionDefinition clang::Parser::ParseDeclGroup...