Clean up some code in Program.

Clean up some code in Program.

NFC here, this just raises some platform specific ifdef hackery
out of a class and creates proper platform-independent typedefs
for the relevant things.  This allows these typedefs to be
reused in other places without having to reinvent this preprocessor
logic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334294 91177308-0d34-0410-b5e6-96231b3b80d8