LLVM 3.0's raw_fd_ostream doesn't take an std::string as file name.

Bug swiftshader:123
Bug b/123193054

Change-Id: I994f6fe5ce98c573d4be48737934b7c6c0127434
Reviewed-on: https://swiftshader-review.googlesource.com/c/24088Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>