[dllimport] Honor always_inline when deciding whether a dllimport function...

[dllimport] Honor always_inline when deciding whether a dllimport function should be available for inlining (PR48925)

Normally, Clang will not make dllimport functions available for inlining
if they reference non-imported symbols, as this can lead to confusing
link errors. But if the function is marked always_inline, the user
presumably knows what they're doing and the attribute should be honored.

Differential revision: https://reviews.llvm.org/D95673