Drop 'dllimport' when redeclaring inline function template without the attribute (PR31695)

For non-template dllimport functions, MSVC allows providing an inline
definition without spelling out the attribute again. In the example below, f
remains a dllimport function.

  __declspec(dllimport) int f();
  inline int f() { return 42; }

  int useit() {
    return f();
  }

However, for a function template, not putting dllimport on the redeclaration
causes it to be dropped. In the example below, f is not dllimport.

  template <typename> __declspec(dllimport) int f();
  template <typename> inline int f() { return 42; }

  int useit() {
    return f<int>();
  }

This patch makes Clang match MSVC for the second example.

MSVC does not warn about the attribute being dropped in the example above, but
I think we should. (MSVC does warn if the inline keyword isn't used.)

Differential Revision: https://reviews.llvm.org/D29152

llvm-svn: 293800