Fix all the CMake code that can only handle -stdlib= but not --stdlib=

[teemperor]

Summary

There are several places in LLVM's CMake setup that try to remove the stdlib=... flag from the
CMake flags. All this code however only considered the -stdlib= variant of the flag but not
the alternative spelling with a double dash. This causes that when one adds --stdlib=...
to the user-provided CMake flags that this gets transformed into just - which ends up causing the
build system to think it should read the source from stdin (which then lead to very confusing build
errors).

This just adds the alternative spelling before the`-stdlib=` variant in all these places

[ldionne]

This LGTM, however it is an artifact of the fact that LLVM sets flags globally, and we then need to sanitize them. In the longer term, it would be nice to completely stop setting flags globally in LLVM, or to insulate the runtimes from those globally set flags somehow.

libcxx/CMakeLists.txt

 
 include(HandleLibCXXABI) # Setup the ABI library flags
 
 if (NOT LIBCXX_STANDALONE_BUILD)
   # Remove flags that may have snuck in.
   remove_flags(-DNDEBUG -UNDEBUG -D_DEBUG
                -lc++abi)
 endif()
-remove_flags(-stdlib=libc++ -stdlib=libstdc++)
+remove_flags(--stdlib=libc++ -stdlib=libc++ --stdlib=libstdc++ -stdlib=libstdc++)
 
 # FIXME: Remove all debug flags and flags that change which Windows
 # default libraries are linked. Currently we only support linking the
 # non-debug DLLs
 remove_flags("/D_DEBUG" "/MTd" "/MDd" "/MT" "/Md")
 
 # FIXME(EricWF): See the FIXME on LIBCXX_ENABLE_PEDANTIC.
 # Remove the -pedantic flag and -Wno-pedantic and -pedantic-errors

libcxxabi/CMakeLists.txt

if (LIBCXXABI_HAS_NOSTDINCXX_FLAG)
   # cmake 3.14 and above remove system include paths that are explicitly
   # passed on the command line.  We build with -nostdinc++ and explicitly add
   # just the libcxx system include paths with -I on the command line.
   # Setting CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES effectively prevents cmake
   # from removing these.
   # See: https://gitlab.kitware.com/cmake/cmake/issues/19227
   set(CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES "")
   # Remove -stdlib flags to prevent them from causing an unused flag warning.
+  string(REPLACE "--stdlib=libc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+  string(REPLACE "--stdlib=libstdc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REPLACE "-stdlib=libc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REPLACE "-stdlib=libstdc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 endif()
 
 # Let the library headers know they are currently being used to build the
 # library.
 add_definitions(-D_LIBCXXABI_BUILDING_LIBRARY)
 

libunwind/CMakeLists.txt

 
 add_cxx_compile_flags_if_supported(-fno-exceptions)
 add_cxx_compile_flags_if_supported(-fno-rtti)
 
 # Ensure that we don't depend on C++ standard library.
 if (LIBUNWIND_HAS_NOSTDINCXX_FLAG)
   list(APPEND LIBUNWIND_COMPILE_FLAGS -nostdinc++)
   # Remove -stdlib flags to prevent them from causing an unused flag warning.
+  string(REPLACE "--stdlib=libc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+  string(REPLACE "--stdlib=libstdc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REPLACE "-stdlib=libc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REPLACE "-stdlib=libstdc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 endif()
 
 # Assert
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 if (LIBUNWIND_ENABLE_ASSERTIONS)
   # MSVC doesn't like _DEBUG on release builds. See PR 4379.