Check for LLVM_ON_WIN32 instead of _WIN32.

This is a follow-up to r203624 to address Anton's comment.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203668 91177308-0d34-0410-b5e6-96231b3b80d8
Don't recompute inlining threshold - use new overload provided by LLVM

Follow-up on LLVM r203669


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203673 91177308-0d34-0410-b5e6-96231b3b80d8