Move std::function constructor SFINAE into template parameter list. Fixes PR20002.
Move std::function constructor SFINAE into template parameter list. Fixes PR20002.

Although inheriting constructors have already been fixed in Clang 3.9 I still
choose to fix std::function so users can derive from it with older compilers.


git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@276090 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx