[CMake] Differentiate between static and shared libc++abi

This addresses the issue introduced in r354212 which broke the case when
static libc++abi is merged into static libc++, but shared libc++ is
linked against shared libc++. There are 4 different possible
combinations which is difficult to capture using a single variable. This
change splits LIBCXX_CXX_ABI_LIBRARY into two:
LIBCXX_CXX_SHARED_ABI_LIBRARY and LIBCXX_CXX_STATIC_ABI_LIBRARY to
handle the shared and static cases. This in turn allows simplification
of some of the logic around merging of static archives.

Differential Revision: https://reviews.llvm.org/D60114

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357556 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed