Revert r359048: C++ DR2387: a variable template declared wthi

Revert r359048: C++ DR2387: a variable template declared wthi

The change breaks libc++ with the follwing error:

In file included from valarray:4:
.../include/c++/v1/valarray:1062:60: error: explicit instantiation declaration of 'valarray<_Tp>' with internal linkage
_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS valarray<size_t>::valarray(size_t))
                                                           ^
.../include/c++/v1/valarray:1063:60: error: explicit instantiation declaration of '~valarray<_Tp>' with internal linkage
_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS valarray<size_t>::~valarray())

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@359076 91177308-0d34-0410-b5e6-96231b3b80d8