[clang] [libc] [libcxx] [libc][libcxx] Support for building libc++ against LLVM libc (PR #99287)

[Petr Hosek via cfe-commits]

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@@ -784,6 +794,10 @@ config_define_if_not(LIBCXX_ENABLE_WIDE_CHARACTERS _LIBCPP_HAS_NO_WIDE_CHARACTER
 config_define_if_not(LIBCXX_ENABLE_TIME_ZONE_DATABASE _LIBCPP_HAS_NO_TIME_ZONE_DATABASE)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 
+if (LIBCXX_LIBC STREQUAL "llvm-libc")
+  config_define(ON _LIBCPP_HAS_LLVM_LIBC)
+endif()
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petrhosek wrote:

I removed these and will address it more correctly in a follow up PR.

https://github.com/llvm/llvm-project/pull/99287