[libc++] Build the dylib with -faligned-allocation

[ldionne]

Summary

This avoids issues when building the dylib for deployment targets that
don't support aligned allocation, where Clang normally triggers an
error to warn users their code would break at runtime when back-deployed.
Since we're building the dylib itself, which contains the aligned
allocation functions, we don't want to trigger that error.

[ldionne]

@dmajor @froydnj Can you please pull this downstream and tell me if that fixes your problem? It should.

[dmajor]

Kicked off a run.

[dmajor]

Our mac builds are back to green, and no complaints from the other platforms. Thank you!

libcxx/CMakeLists.txt

function(cxx_add_basic_build_flags target)
 
   # Require C++17 for all targets. C++17 is needed to use aligned allocation
   # in the dylib.
   set_target_properties(${target} PROPERTIES
     CXX_STANDARD 17
     CXX_STANDARD_REQUIRED YES
     CXX_EXTENSIONS NO)
 
+  # When building the dylib, don't warn for unavailable aligned allocation
+  # functions based on the deployment target -- they are always available
+  # because they are provided by the dylib itself.
+  target_add_compile_flags_if_supported(${target} PRIVATE -faligned-allocation)
+
   # On all systems the system c++ standard library headers need to be excluded.
   # MSVC only has -X, which disables all default includes; including the crt.
   # Thus, we do nothing and hope we don't accidentally include any of the C++
   # headers
   target_add_compile_flags_if_supported(${target} PUBLIC -nostdinc++)
 
   # Hide all inline function definitions which have not explicitly been marked
   # visible. This prevents new definitions for inline functions from appearing in