[libc++abi] Default to DLL annotations on Windows
`__declspec(dllexport)` and `__declspec(dllimport)` should only be used
when building libc++abi as a DLL, but that's the more common use case,
so default to adding the annotations and add an option to opt out.
Similar to r282449, which made the corresponding change for libc++.
Differential Revision: https://reviews.llvm.org/D24945
git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@282470 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed