[runtimes] [CMake] Fix checks for -Werror when building with incomplete toolchains

When we add `--unwindlib=none` during the CMake configure phase (to
make CMake linking tests succeed before the unwind library has been
built for the first time - when bootstrapping a cross toolchain from
scratch), we add it to `CMAKE_REQUIRED_FLAGS` to make later CMake tests
pass.

When the option is added to `CMAKE_REQUIRED_FLAGS`, it gets added to
both compilation and linking commands. When --unwindlib=none is added
to the compilation command, it causes warnings (about being unused
during compilation, as it only affects linking).

When all CMake test compilations produce warnings, later CMake tests
for `-Werror` fail.

Add `--{start,end}-no-unused-arguments` around `--unwindlib=none`, if
supported, to avoid unnecessary warnings due to this option.

If the CMake requirement is bumped to 3.14, we could use
`CMAKE_REQUIRED_LINK_OPTIONS` instead, removing the need for the
`--{start,end}-no-unused-arguments` options. (However, do note that
`CMAKE_REQUIRED_LINK_OPTIONS` is problematic in combination with
`CMAKE_TRY_COMPILE_TARGET_TYPE` set to `STATIC_LIBRARY`; see
https://gitlab.kitware.com/cmake/cmake/-/issues/23454.)

Differential Revision: https://reviews.llvm.org/D124377

Author: mstorsjo

runtimes/CMakeLists.txt

@@ -102,7 +102,27 @@ if (NOT LLVM_RUNTIMES_LINKING_WORKS)
   # e.g. ASAN/TSAN.
   llvm_check_compiler_linker_flag(C "--unwindlib=none" CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG)
   if (CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG)
+    set(ORIG_CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --unwindlib=none")
+    # TODO: When we can require CMake 3.14, we should use
+    # CMAKE_REQUIRED_LINK_OPTIONS here. Until then, we need a workaround:
+    # When using CMAKE_REQUIRED_FLAGS, this option gets added both to
+    # compilation and linking commands. That causes warnings in the
+    # compilation commands during cmake tests. This is normally benign, but
+    # when testing whether -Werror works, that test fails (due to the
+    # preexisting warning).
+    #
+    # Therefore, before we can use CMAKE_REQUIRED_LINK_OPTIONS, check if we
+    # can use --start-no-unused-arguments to silence the warnings about
+    # --unwindlib=none during compilation.
+    #
+    # We must first add --unwindlib=none to CMAKE_REQUIRED_FLAGS above, to
+    # allow this subsequent test to succeed, then rewrite CMAKE_REQUIRED_FLAGS
+    # below.
+    check_c_compiler_flag("--start-no-unused-arguments" C_SUPPORTS_START_NO_UNUSED_ARGUMENTS)
+    if (C_SUPPORTS_START_NO_UNUSED_ARGUMENTS)
+      set(CMAKE_REQUIRED_FLAGS "${ORIG_CMAKE_REQUIRED_FLAGS} --start-no-unused-arguments --unwindlib=none --end-no-unused-arguments")
+    endif()
   endif()
 endif()