Use a two-level cast through an intptr_t, and make them C-style casts.

Use a two-level cast through an intptr_t, and make them C-style casts.
This shouldn't have any functional difference, but it appears to be the
pattern used for other methods on DynamicLibrary, and it should avoid
the -Wpedantic warning on one of the build bots about the direct
reinterpret_cast.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272461 91177308-0d34-0410-b5e6-96231b3b80d8