[libcxxabi][CMake] Drop unused HandleOutOfTreeLLVM include

[phosek]

Summary

This include doesn't seem to be needed for the standalone build (it's
not being used by libc++ build either), but introduces unnecessary
dependency because HandleOutOfTreeLLVM performs checks that require
a working C++ library. We shouldn't require a working C++ library to
build libc++abi or libc++ (it's what we're building after all).

[phosek]

This is an alternative to D58331 which is an attempt at resolving the breakage introduced in D58013. I think this is a better solution than D58331. Do you know if there's any reason why HandleLLVMOptions was included here?

[Hahnfeld]

In D58333#1400699, @phosek wrote:

This is an alternative to D58331 which is an attempt at resolving the breakage introduced in D58013. I think this is a better solution than D58331.

I also think that this one is better than D58331, for the reasons stated in the summary.

Do you know if there's any reason why HandleLLVMOptions was included here?

HandleLLVMOptions seems to setup quite a few things, but I think most of them don't apply for libc++abi and libc++ (like LTO, assertions, -std=c++11). From looking over the code I guess that warnings and LLVM_USE_LINKER could be relevant?

[sbc100]

Thank you. I'd been try to land this myself (See discussion in https://reviews.llvm.org/D33753).

libcxxabi/trunk/cmake/Modules/HandleOutOfTreeLLVM.cmake

if (WIN32)
       set(LLVM_ON_UNIX 0)
       set(LLVM_ON_WIN32 1)
     else()
       set(LLVM_ON_UNIX 1)
       set(LLVM_ON_WIN32 0)
     endif()
   endif()
   if (LLVM_FOUND)
-    # Enable warnings, otherwise -w gets added to the cflags by HandleLLVMOptions.
-    set(LLVM_ENABLE_WARNINGS ON)
     include(AddLLVM OPTIONAL)
-    include(HandleLLVMOptions OPTIONAL)
   endif()
 
   # LLVM Options --------------------------------------------------------------
   if (NOT DEFINED LLVM_INCLUDE_TESTS)
     set(LLVM_INCLUDE_TESTS ${LLVM_FOUND})
   endif()
   if (NOT DEFINED LLVM_INCLUDE_DOCS)
     set(LLVM_INCLUDE_DOCS ${LLVM_FOUND})