constexpr: semantic checking for constexpr variables.

constexpr: semantic checking for constexpr variables.

We had an extension which allowed const static class members of floating-point type to have in-class initializers, 'as a C++0x extension'. However, C++0x does not allow this. The extension has been kept, and extended to all literal types in C++0x mode (with a fixit to add the 'constexpr' specifier).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140801 91177308-0d34-0410-b5e6-96231b3b80d8