[globalisel][tablegen] Fix an issue with lambdas when compiling with older GCC's

It seems that G++ 4.8 doesn't accept the 'enum A' in code of the form:
  enum A { ... };
  const auto &F = []() -> enum A { ... };
However, it does accept:
  typedef enum { ... } A;
  const auto &F = []() -> A { ... };

llvm-svn: 308599