Remove one more warning from clang-cl build

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@343440 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3ea86ba..89c3536 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -567,6 +567,7 @@
         -Wno-old-style-cast
         -Wno-deprecated # FIXME: Remove this and fix all occurrences.
         -Wno-shift-sign-overflow # FIXME: Why do we need this with clang-cl but not clang?
+        -Wno-double-promotion # FIXME: remove me
       )
     endif()
 elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")