[llvm] Remember LLVM_ENABLE_LIBCXX setting in installed configuration (PR #134990)

[David Spickett via llvm-commits]

DavidSpickett wrote:

Unfortunately still not working for flang: https://lab.llvm.org/buildbot/#/builders/89/builds/20320

Various headers are not found, kinda like the failure reported above but more of them:
```
FAILED: flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/allocator-registry.cpp.o 
/home/tcwg-buildbot/worker/flang-aarch64-libcxx/build/./bin/clang++ --target=aarch64-unknown-linux-gnu -D_DEBUG -D_GLIBCXX_ASSERTIONS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang-rt/include -I/home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang-rt/../flang/include -I/home/tcwg-buildbot/worker/flang-aarch64-libcxx/build/runtimes/runtimes-bins/flang-rt -stdlib=libc++ -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=gnu++17 -UNDEBUG -fno-lto -fno-exceptions -fno-rtti -funwind-tables -fno-asynchronous-unwind-tables -U_GLIBCXX_ASSERTIONS -U_LIBCPP_ENABLE_ASSERTIONS -MD -MT flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/allocator-registry.cpp.o -MF flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/allocator-registry.cpp.o.d -o flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/allocator-registry.cpp.o -c /home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang-rt/lib/runtime/allocator-registry.cpp
In file included from /home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang-rt/lib/runtime/allocator-registry.cpp:9:
/home/tcwg-buildbot/worker/flang-aarch64-libcxx/llvm-project/flang-rt/include/flang-rt/runtime/allocator-registry.h:14:10: fatal error: 'cstdint' file not found
   14 | #include <cstdint>
      |          ^~~~~~~~~
```
I can test things locally if needed.

https://github.com/llvm/llvm-project/pull/134990