Don't dllimport inline functions when targeting MinGW (PR21366)

It turns out that MinGW never dllimports of exports inline functions.
This means that code compiled with Clang would fail to link with
MinGW-compiled libraries since we might try to import functions that
are not imported.

To fix this, make Clang never dllimport inline functions when targeting
MinGW.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221154 91177308-0d34-0410-b5e6-96231b3b80d8