Reduce -Wregister from DefaultError to a default-on warning.

[jyknight]

Summary

There is a lot of C++ code in the wild still using 'register', which
will become broken if we switch the default compilation mode to
C++17. A default-on warning seems to get the message across
sufficiently, without unnecessarily breaking code that hasn't asked
for it (e.g. with -Werror). GCC has also chosen to make -Wregister an
on-by-default warning.

clang/include/clang/Basic/DiagnosticSemaKinds.td

def warn_auto_storage_class : Warning<
   "'auto' storage class specifier is redundant and incompatible with C++11">,
   InGroup<CXX11Compat>, DefaultIgnore;
 
 def warn_deprecated_register : Warning<
   "'register' storage class specifier is deprecated "
   "and incompatible with C++17">, InGroup<DeprecatedRegister>;
 def ext_register_storage_class : ExtWarn<
   "ISO C++17 does not allow 'register' storage class specifier">,
-  DefaultError, InGroup<Register>;
+  InGroup<Register>;
 
 def err_invalid_decl_spec_combination : Error<
   "cannot combine with previous '%0' declaration specifier">;
 def err_invalid_vector_decl_spec_combination : Error<
   "cannot combine with previous '%0' declaration specifier. "
   "'__vector' must be first">;
 def err_invalid_pixel_decl_spec_combination : Error<
   "'__pixel' must be preceded by '__vector'.  "

clang/test/SemaCXX/deprecated.cpp

 #elif __cplusplus >= 201103L
 // expected-warning@-8 {{dynamic exception specifications are deprecated}} expected-note@-8 {{use 'noexcept' instead}}
 // expected-warning@-8 {{dynamic exception specifications are deprecated}} expected-note@-8 {{use 'noexcept(false)' instead}}
 // expected-warning@-8 {{dynamic exception specifications are deprecated}} expected-note@-8 {{use 'noexcept(false)' instead}}
 #endif
 
 void stuff(register int q) {
 #if __cplusplus > 201402L
-  // expected-error@-2 {{ISO C++17 does not allow 'register' storage class specifier}}
+  // expected-warning@-2 {{ISO C++17 does not allow 'register' storage class specifier}}
 #elif __cplusplus >= 201103L && !defined(NO_DEPRECATED_FLAGS)
   // expected-warning@-4 {{'register' storage class specifier is deprecated}}
 #endif
   register int n;
 #if __cplusplus > 201402L
-  // expected-error@-2 {{ISO C++17 does not allow 'register' storage class specifier}}
+  // expected-warning@-2 {{ISO C++17 does not allow 'register' storage class specifier}}
 #elif __cplusplus >= 201103L && !defined(NO_DEPRECATED_FLAGS)
   // expected-warning@-4 {{'register' storage class specifier is deprecated}}
 #endif
 
   register int m asm("rbx"); // no-warning
 
   int k = to_int(n); // no-warning
   bool b;