[libc++] Hoist extern template above first use
[libc++] Hoist extern template above first use

This function template is referenced inside class basic_string as a
friend function. The extern template declaration needs to be above that
friend declaration to actually take effect.

This is important because this function was marked as exported in
r307966, so without the extern template taking effect, it can leak into
other DSOs as a visible symbol.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@309474 91177308-0d34-0410-b5e6-96231b3b80d8

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