Fix build for mingw.

Summary: I was building lldb using cross mingw-w64 toolchain on Linux and observed some issues. This is first patch in the series to fix that build. It mostly corrects the case of include files and adjusts some #ifdefs from _MSC_VER to _WIN32 and vice versa. I built lldb on windows with VS after applying this patch to make sure it does not break the build there.

Reviewers: zturner, labath, abidh

Subscribers: ki.stfu, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D27759

llvm-svn: 289821