This is likely unrelated to ide-cpp, but I will post here to have any advice...

I've installed ide-cpp on Atom on macOS Mojave 10.14.5.
clangd is installed with brew (brew install llvm) under:

/usr/local/Cellar/llvm/8.0.0_1/bin/clangd

I have added this path to the ide-cpp. Everything works, expect the diagnostic, where I get a similar problem reported at onivim/oni#826 Unfortunately, what it is suggested there doesn't work on Mojave anymore (and it is was a real hack).

From my understanding, there is a conflict between headers of different compilers, likely the ones installed from Apple under /Library/Developer/CommandLineTools/usr/include/c++/v1 and the version installed with brew under /usr/local/Cellar/llvm/8.0.0_1/include/c++/v1/