Improve the semantic checking for explicit instantiations of

Improve the semantic checking for explicit instantiations of
templates. In particular:
  - An explicit instantiation can follow an implicit instantiation (we
  were improperly diagnosing this as an error, previously).
  - In C++0x, an explicit instantiation that follows an explicit
  specialization of the same template specialization is ignored. In
  C++98, we just emit an extension warning.
  - In C++0x, an explicit instantiation must be in a namespace
  enclosing the original template. C++98 has no such requirement.

Also, fixed a longstanding FIXME regarding the integral type that is
used for the size of a constant array type when it is being instantiated.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@71689 91177308-0d34-0410-b5e6-96231b3b80d8