[compiler-rt] suppress LNK4221 warning when creating static libraries

[inglorion]

Summary

We put empty object files in archives, which causes MSVC's linker to
complain about these objects not defining any previously undefined
symbols. Since we do it on purpose, this only creates noise during
the build process. This patch causes us to suppress the warnings.

[rnk]

lgtm

CMakeLists.txt

229–231

I'd simplify the first two sentences to something like:
Silence MSVC linker warning about empty object files. The sanitizer libraries intentionally use ifdefs to compile out entire files, rather than skipping them in the build system. Ideally, ...

The reader should already know that we aim to have zero warnings and that we silence noisy warnings.

[inglorion]

Reworded comment as per @rnk's suggestion.

[inglorion]

Remove changes that were unintentionally pulled in.

[inglorion]

One more time...

CMakeLists.txt

 append_list_if(COMPILER_RT_HAS_WD4291_FLAG /wd4291 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4391_FLAG /wd4391 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4722_FLAG /wd4722 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4800_FLAG /wd4800 SANITIZER_COMMON_CFLAGS)
 
 # Warnings to turn off for all libraries, not just sanitizers.
 append_string_if(COMPILER_RT_HAS_WUNUSED_PARAMETER_FLAG -Wno-unused-parameter CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
+if (CMAKE_LINKER MATCHES "link.exe$")
+  # We put empty object files in archives, which causes MSVC's linker to
+  # complain about these objects not defining any previously undefined
+  # symbols. Since we do it on purpose, this only creates noise during
+  # the build process, so we suppress the warnings here. Ideally, we would
+  # pass this flag only for the libraries that need it, but CMake doesn't
+  # seem to have a way to set linker flags for individual static libraries,
+  # so we enable the suppression flag for the whole compiler-rt project.
+  append("/IGNORE:4221" CMAKE_STATIC_LINKER_FLAGS)
+endif()
+
 add_subdirectory(include)
 
 set(COMPILER_RT_LIBCXX_PATH ${LLVM_MAIN_SRC_DIR}/projects/libcxx)
 if(EXISTS ${COMPILER_RT_LIBCXX_PATH}/)
   set(COMPILER_RT_HAS_LIBCXX_SOURCES TRUE)
 else()
   set(COMPILER_RT_HAS_LIBCXX_SOURCES FALSE)
 endif()