Add an attribute to forbid temporary instances of a type. This allows class

authors to write 

  class __attribute__((forbid_temporaries)) Name { ... };

when they want to force users to name all variables of the type. This protects
people from doing things like creating a scoped_lock that only lives for a
single statement instead of an entire scope.

The warning produced by this attribute can be disabled by
-Wno-forbid-temporaries.

llvm-svn: 124217