Remember LLVM_ENABLE_LIBCXX setting in installed configuration (#134990)

The buidbot
[flang-aarch64-libcxx](https://lab.llvm.org/buildbot/#/builders/89) is
currently failing with an ABI issue. The suspected reason is that
LLVMSupport.a is built using libc++, but the unittests are using the
default C++ standard library, libstdc++ in this case. This predefined
`llvm_gtest` target uses the LLVMSupport from `find_package(LLVM)`,
which finds the libc++-built LLVMSupport.

To fix, store the `LLVM_ENABLE_LIBCXX` setting in the LLVMConfig.cmake
such that everything that links to LLVM libraries use the same standard
library. In this case discussed in
llvm/llvm-zorg#387 it was the flang-rt
unittests, but other runtimes with GTest unittests should have the same
issue (e.g. offload), and any external project that uses
`find_package(LLVM)`.

Author: Meinersbur

llvm/cmake/modules/LLVMConfig.cmake.in

@@ -55,6 +55,8 @@ endif()
 
 set(LLVM_ENABLE_RTTI @LLVM_ENABLE_RTTI@)
 
+set(LLVM_ENABLE_LIBCXX @LLVM_ENABLE_LIBCXX@)
+
 set(LLVM_ENABLE_LIBEDIT @HAVE_LIBEDIT@)
 if(LLVM_ENABLE_LIBEDIT)
   find_package(LibEdit)