[CMake] Add bootstrap option `CLANG_BOOTSTRAP_USE_LIBCXX`

[aoli]

Summary

This option makes clang-bootstrap-deps depend on libcxx taret. It
allows stage2 build to link against libcxx libraries from stage1 build.

[beanz]

I'm not sure this change is sufficient to do what you're intending. I think you also need to pass the library path of the cxx library into the bootstrap configuration otherwise it will use the one from the sysroot.

[aoli]

Pass cxx library path into the bootstrap configuration.

[aoli]

Adding LLVM_LIBRARY_DIR to rpath can solve this problem. But I'm not sure if it will have any side effects.

CMakeLists.txt

set(_BOOTSTRAP_DEFAULT_PASSTHROUGH
     CMAKE_OSX_ARCHITECTURES)
 
   # We don't need to depend on compiler-rt if we're building instrumented
   # because the next stage will use the same compiler used to build this stage.
   if(TARGET compiler-rt AND NOT LLVM_BUILD_INSTRUMENTED)
     add_dependencies(clang-bootstrap-deps compiler-rt)
   endif()
 
+  if(CLANG_BOOTSTRAP_USE_LIBCXX)
+    add_dependencies(clang-bootstrap-deps cxx)
+    list(APPEND BOOTSTRAP_CMAKE_INSTALL_RPATH "${LLVM_LIBRARY_DIR}")
+    set(BOOTSTRAP_CMAKE_BUILD_WITH_INSTALL_RPATH TRUE)
+  endif()
+
   set(C_COMPILER "clang")
   set(CXX_COMPILER "clang++")
   if(WIN32)
     set(C_COMPILER "clang-cl.exe")
     set(CXX_COMPILER "clang-cl.exe")
   endif()
 
   set(COMPILER_OPTIONS

cmake/caches/Android.cmake

 # This file sets up a CMakeCache for an Android toolchain build.
 
 set(LLVM_TARGETS_TO_BUILD X86 CACHE STRING "")
 
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
 set(CLANG_ENABLE_STATIC_ANALYZER OFF CACHE BOOL "")
 set(CLANG_VENDOR Android CACHE STRING "")
+set(CLANG_BOOTSTRAP_USE_LIBCXX ON CACHE BOOL "")
 
 set(CMAKE_BUILD_TYPE RELEASE CACHE STRING "")
 
 set(HAVE_LIBCXXABI ON CACHE BOOL "")
 set(LLVM_BUILD_TOOLS OFF CACHE BOOL "")
 set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
 set(LLVM_ENABLE_THREADS OFF CACHE BOOL "")
 set(LLVM_LIBDIR_SUFFIX 64 CACHE STRING "")