Clangd diagnostics can't include stl headers on Mac

[maciejzj]

I have installed clangd with :LspInstallServer but the diagnostic throws errors when I include any standard library files.
#include <string> results in:

main.cpp|1 col 10| clang:Error:pp_file_not_found:In included file: 'string.h' file not found /Users/maciej/.vim/bundle/vim-lsp-settings/servers/clangd/bin/../include/c++/v1/string.h:60:15: note: error     occurred here 

I took a look at the string in the .vim/bundle/vim-lsp-settings/servers/clangd/bin/../include/c++/v1/ and it fails at:

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
#include_next <string.h>

with the diagnostics:

string.h|57 col 13| clang:Warning:-Wpragma-system-header-outside-header:#pragma system_header ignored in main file
string.h|57 col 13| clang:Warning:-Wpragma-system-header-outside-header:#pragma system_header ignored in main file
string.h|60 col 15| clang:Error:pp_including_mainfile_in_preamble:Main file cannot be included recursively when building a preamble

I am not sure whether it is my fault, something wrong with the vim-lip-settings or with the llvm code that is downloaded with the :LspInstallServer, but the c++ diagnostics doesn't work out of the box.

Also it results in very superficial autocompletion of stl, because the clangd can't parse the library and does not complete methods/members of classes.

[kjuq]

It's caused by macos catalina removing /usr/include.
Clang 9.0 looks for header files in the path, but there is no such path.
Thus I updated clangd. The problem will be resolved after my PR: #210 accepted.