Replace "int n = str_.size();" with "int n = static_cast<int>(str_.size());".
int is the correct type to use, because we're eventually calling
"base::pbump(n+1);" where base is std::basic_streambuf.
N4606 27.6.3.3.3 [streambuf.put.area]/4 declares: "void pbump(int n);"
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@288751 91177308-0d34-0410-b5e6-96231b3b80d8
