[libunwind] Install the headers by default

Just like we install libc++ and libc++abi headers by default when we
install the library, it makes sense to install the libunwind headers
by default when we build libunwind. In the current state of things,
there is an increased risk that folks are using older (previously
installed) libunwind headers along with a recent libunwind dylib,
which is not ideal.

Differential Revision: https://reviews.llvm.org/D135663

Author: ldionne

libunwind/CMakeLists.txt

@@ -49,7 +49,7 @@ option(LIBUNWIND_INCLUDE_TESTS "Build the libunwind tests." ${LLVM_INCLUDE_TESTS
 option(LIBUNWIND_IS_BAREMETAL "Build libunwind for baremetal targets." OFF)
 option(LIBUNWIND_USE_FRAME_HEADER_CACHE "Cache frame headers for unwinding. Requires locking dl_iterate_phdr." OFF)
 option(LIBUNWIND_REMEMBER_HEAP_ALLOC "Use heap instead of the stack for .cfi_remember_state." OFF)
-option(LIBUNWIND_INSTALL_HEADERS "Install the libunwind headers." OFF)
+option(LIBUNWIND_INSTALL_HEADERS "Install the libunwind headers." ON)
 
 set(LIBUNWIND_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
     "Define suffix of library directory name (32/64)")