Parse: Don't attempt to act on #pragma init_seg when not targeting MSVC

It doesn't really make sense to try and do stuff with #pragma init_seg
when targeting non-Microsoft platforms; notions like library vs user
initializers don't exist for other targets.

This fixes PR20639.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215618 91177308-0d34-0410-b5e6-96231b3b80d8