[Windows] Remove the #include <eh.h> hack.
[Windows] Remove the #include <eh.h> hack.

Prior to MSVC 2015 we had to manually include this header any
time we were going to include <thread> or <future> due to a
bug in MSVC's STL implementation.  This has been fixed in MSVC
for some time now, and we require VS 2015 minimum, so we can
remove this across all subprojects.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@296906 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld