Revert "[runtimes] Fix building initial libunwind+libcxxabi+libcxx with compiler implied -lunwind"

This reverts commit 7c3d19ab7b.

This commit was reported as causing build problems for the amdgpu
buildbot in https://reviews.llvm.org/D113253#3137097.

libcxx/CMakeLists.txt

@@ -773,13 +773,6 @@ function(cxx_link_system_libraries target)
     target_add_link_flags_if_supported(${target} PRIVATE "/nodefaultlib")
   endif()
 
-  if (LIBCXX_SUPPORTS_UNWINDLIB_NONE_FLAG AND LIBCXXABI_USE_LLVM_UNWINDER)
-    # If we're linking directly against the libunwind that we're building
-    # in the same invocation, don't try to link in the toolchain's
-    # default libunwind (which may be missing still).
-    target_add_link_flags_if_supported(${target} PRIVATE "--unwindlib=none")
-  endif()
-
   if (LIBCXX_HAS_SYSTEM_LIB)
     target_link_libraries(${target} PRIVATE System)
   endif()

libcxx/cmake/config-ix.cmake

@@ -1,22 +1,9 @@
 include(CMakePushCheckState)
 include(CheckLibraryExists)
-include(CheckLinkerFlag)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckCSourceCompiles)
 
-# The compiler driver may be implicitly trying to link against libunwind.
-# This is normally ok (libcxx relies on an unwinder), but if libunwind is
-# built in the same cmake invocation as libcxx and we've got
-# LIBCXXABI_USE_LLVM_UNWINDER set, we'd be linking against the just-built
-# libunwind (and the compiler implicit -lunwind wouldn't succeed as the newly
-# built libunwind isn't installed yet). For those cases, it'd be good to
-# link with --uwnindlib=none. Check if that option works.
-llvm_check_linker_flag("--unwindlib=none" LIBCXX_SUPPORTS_UNWINDLIB_NONE_FLAG)
-if (LIBCXX_SUPPORTS_UNWINDLIB_NONE_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --unwindlib=none")
-endif()
-
 if(WIN32 AND NOT MINGW)
   # NOTE(compnerd) this is technically a lie, there is msvcrt, but for now, lets
   # let the default linking take care of that.

runtimes/CMakeLists.txt

@@ -1,21 +1,6 @@
 # This file handles building LLVM runtime sub-projects.
 cmake_minimum_required(VERSION 3.13.4)
-
+project(Runtimes C CXX ASM)
-# Add path for custom and the LLVM build's modules to the CMake module path.
-list(INSERT CMAKE_MODULE_PATH 0
-  "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
-  "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
-  "${CMAKE_CURRENT_SOURCE_DIR}/../cmake"
-  "${CMAKE_CURRENT_SOURCE_DIR}/../cmake/Modules"
-  "${CMAKE_CURRENT_SOURCE_DIR}/../llvm/cmake"
-  "${CMAKE_CURRENT_SOURCE_DIR}/../llvm/cmake/modules"
-)
-
-# We may have an incomplete toolchain - do language support tests without
-# linking.
-include(EnableLanguageNolink)
-project(Runtimes LANGUAGES NONE)
-llvm_enable_language_nolink(C CXX ASM)
 
 set(LLVM_ALL_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
@@ -43,6 +28,14 @@ endfunction()
 find_package(LLVM PATHS "${LLVM_BINARY_DIR}" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
 find_package(Clang PATHS "${LLVM_BINARY_DIR}" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
 
+# Add path for custom and the LLVM build's modules to the CMake module path.
+list(INSERT CMAKE_MODULE_PATH 0
+  "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
+  "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
+  "${CMAKE_CURRENT_SOURCE_DIR}/../llvm/cmake"
+  "${CMAKE_CURRENT_SOURCE_DIR}/../llvm/cmake/modules"
+)
+
 function(get_compiler_rt_path path)
   foreach(entry ${runtimes})
     get_filename_component(projName ${entry} NAME)
@@ -93,29 +86,14 @@ if(APPLE)
 endif()
 
 include(CheckLibraryExists)
-include(CheckLinkerFlag)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
-
-check_c_compiler_flag("" LLVM_RUNTIMES_LINKING_WORKS)
-if (NOT LLVM_RUNTIMES_LINKING_WORKS)
-  # The compiler driver may be implicitly trying to link against libunwind, which
-  # might not work if libunwind doesn't exist yet. Try to check if
-  # --unwindlib=none is supported, and use that if possible.
-  # Don't add this if not necessary to fix linking, as it can break using
-  # e.g. ASAN/TSAN.
-  llvm_check_linker_flag("--unwindlib=none" LLVM_RUNTIMES_SUPPORT_UNWINDLIB_NONE_FLAG)
-  if (LLVM_RUNTIMES_SUPPORT_UNWINDLIB_NONE_FLAG)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --unwindlib=none")
-  endif()
-endif()
-
 # Disable use of the installed C++ standard library when building runtimes.
 # Check for -nostdlib++ first; if there's no C++ standard library yet,
 # all check_cxx_compiler_flag commands will fail until we add -nostdlib++
 # (or -nodefaultlibs).
-llvm_check_linker_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
+check_c_compiler_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
 if (LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
 endif()