[CMake] Move cxx-headers to RUNTIME_DISTRIBUTION_COMPONENTS in Apple-stage2.cmake

[ldionne]

Summary

We build libcxx using LLVM_ENABLE_RUNTIMES during Stage2, which requires
cxx-headers to be part of LLVM_RUNTIME_DISTRIBUTION_COMPONENTS instead
of LLVM_DISTRIBUTION_COMPONENTS.

rdar://99028431

[fhahn]

LGTM, thanks!

clang/cmake/caches/Apple-stage2.cmake

 # This file sets up a CMakeCache for Apple-style stage2 bootstrap. It is
 # specified by the stage1 build.
 
-set(LLVM_TARGETS_TO_BUILD X86 ARM AArch64 CACHE STRING "") 
+set(LLVM_TARGETS_TO_BUILD X86 ARM AArch64 CACHE STRING "")
 set(PACKAGE_VENDOR Apple CACHE STRING "")
 set(CLANG_VENDOR_UTI com.apple.clang CACHE STRING "")
 set(LLVM_INCLUDE_EXAMPLES OFF CACHE BOOL "")
 set(LLVM_INCLUDE_DOCS OFF CACHE BOOL "")
 set(LLVM_TOOL_CLANG_TOOLS_EXTRA_BUILD OFF CACHE BOOL "")
 set(CLANG_TOOL_SCAN_BUILD_BUILD OFF CACHE BOOL "")
 set(CLANG_TOOL_SCAN_VIEW_BUILD OFF CACHE BOOL "")
 set(CLANG_LINKS_TO_CREATE clang++ cc c++ CACHE STRING "")
set(LLVM_TOOLCHAIN_UTILITIES
   count
   CACHE STRING "")
 
 set(LLVM_DISTRIBUTION_COMPONENTS
   clang
   LTO
   clang-format
   clang-resource-headers
-  cxx-headers
   Remarks
   ${LLVM_TOOLCHAIN_TOOLS}
   ${LLVM_TOOLCHAIN_UTILITIES}
   CACHE STRING "")
 
+set(LLVM_RUNTIME_DISTRIBUTION_COMPONENTS
+  cxx-headers
+  CACHE STRING "")
+
 # test args
 
 set(LLVM_LIT_ARGS "--xunit-xml-output=testresults.xunit.xml -v" CACHE STRING "")