[gn] Support for building libc++

This change introduces support for building libc++. The library
build should be complete, but not all CMake options have been
replicated in GN. We also don't support tests yet.

We only support two stage build at the moment.

Differential Revision: https://reviews.llvm.org/D61143

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359806 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -138,8 +138,7 @@ set_property(CACHE LIBCXX_CXX_ABI PROPERTY STRINGS ;${CXXABIS})
 # Setup the default options if LIBCXX_CXX_ABI is not specified.
 if (LIBCXX_CXX_ABI STREQUAL "default")
   find_path(
-    LIBCXX_LIBCXXABI_INCLUDES_INTERNAL
-    cxxabi.h
+    LIBCXX_LIBCXXABI_INCLUDES_INTERNAL cxxabi.h
     PATHS ${LLVM_MAIN_SRC_DIR}/projects/libcxxabi/include
           ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi/include
           ${LLVM_MAIN_SRC_DIR}/../libcxxabi/include