Basic: fix warnings from GCC

tools/clang/lib/Basic/DiagnosticIDs.cpp: In function ‘clang::DiagnosticIDs::Level toLevel(clang::diag::Severity)’:
tools/clang/lib/Basic/DiagnosticIDs.cpp:382:1: warning: control reaches end of non-void function [-Wreturn-type]

tools/clang/lib/Format/Format.cpp: In member function ‘virtual std::string clang::format::ParseErrorCategory::message(int) const’:
tools/clang/lib/Format/Format.cpp:282:1: warning: control reaches end of non-void function [-Wreturn-type]

Add a default cases that asserts that we handle the severity, parse error.