Fix filesystem::path assignment from {}

Adding `path::operator=(string_type&&)` made the expression `p = {}`
ambiguous. This path fixes that ambiguity by making the `string&&`
overload a template so it ranks lower during overload resolution.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292345 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed