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

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

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D87133

libcxx/CMakeLists.txt

@@ -478,7 +478,7 @@ if (NOT LIBCXX_STANDALONE_BUILD)
   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

libcxxabi/CMakeLists.txt

@@ -248,6 +248,8 @@ if (LIBCXXABI_HAS_NOSTDINCXX_FLAG)
   # 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()

libunwind/CMakeLists.txt

@@ -321,6 +321,8 @@ add_cxx_compile_flags_if_supported(-fno-rtti)
 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()