Undo turning ExtBehavior into a bitfield.

This produces warnings that I can't explain in a GCC build:

In file included from ../tools/clang/include/clang/Lex/LexDiagnostic.h:13:0,
                 from /usr/local/google/home/djasper/llvm/tools/clang/lib/Lex/PTHLexer.cpp:19:
../tools/clang/include/clang/Basic/Diagnostic.h:219:34: warning: ‘clang::DiagnosticsEngine::DiagState::ExtBehavior’ is too small to hold all values of ‘enum class clang::diag::Severity’ [enabled by default]
     diag::Severity ExtBehavior : 4;     // Map extensions to warnings or errors?
                                  ^

While I don't think this warning makes sense, I'd like to get this back to being
warning-free. This only seems to trigger for "enum class".

Reproducer:
https://godbolt.org/g/P2ekVd

llvm-svn: 302012