PR12848: When emitting a local variable declared 'constexpr', always...

PR12848: When emitting a local variable declared 'constexpr', always initialize it with a store or a memcpy, not by emitting the initializer expression. This is not required for correctness, but more closely aligns with people's expectations, and is cheap (since we've already evaluated the initializer).

llvm-svn: 183082