Suspending neovim causes ALE highlights to be cleared

## Information

**VIM version**

Neovim:
NVIM v0.4.3
Build type: Release

Vim:
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Jan 11 2020 12:16:09)
Included patches: 1-111

Operating System: Linux

## What went wrong

On Neovim (but not Vim), suspending with C-Z causes all ALE highlights to be gone on resume. You have to trigger ALE to run again (reload the file, call :ALELint, etc). Everything is gone: the gutter is no longer open, and nothing is highlighted.

With Vim, all the errors stick around on resume.

With Neovim, switching between buffers doesn't cause this issue, so it's not just when the buffer is hidden. So far the only thing I've seen trigger it is suspending the process.

I'm not sure if it's a Neovim or ALE issue. This doesn't occur with the vim-signify plugin (its gutter remains open on suspend and resume), so that slightly points in ALE's direction, but is obviously not conclusive.

## Reproducing the bug

I pared down my `.vimrc` just enough to set up vim-plug and included *only* ALE as the plugin, to avoid other plugins/settings interfering. I used the same config for both Neovim and Vim.

1. Create a file with an obvious syntax error that ALE will highlight
2. Have ALE run on the file, showing a highlighted problem
3. Suspend Neovim with C-Z
4. Resume Neovim

### :ALEInfo

```
Current Filetype: c
Available Linters: ['ccls', 'clang', 'clangd', 'clangtidy', 'cppcheck', 'cquery', 'flawfinder', 'gcc']
Enabled Linters: ['ccls', 'clang', 'clangd', 'clangtidy', 'cppcheck', 'cquery', 'flawfinder', 'gcc']
Suggested Fixers:
'clang-format' - Fix C/C++ and cuda files with clang-format.
'clangtidy' - Fix C/C++ and ObjectiveC files with clang-tidy.
'remove_trailing_lines' - Remove all blank lines at the end of a file.
'trim_whitespace' - Remove all trailing whitespace characters at the end of every line.
'uncrustify' - Fix C, C++, C#, ObjectiveC, ObjectiveC++, D, Java, Pawn, and VALA files with uncrustify.
Linter Variables:
let g:ale_c_build_dir = ''
let g:ale_c_build_dir_names = ['build', 'bin']
let g:ale_c_ccls_executable = 'ccls'
let g:ale_c_ccls_init_options = {}
let g:ale_c_clang_executable = 'clang'
let g:ale_c_clang_options = '-std=c11 -Wall'
let g:ale_c_clangd_executable = 'clangd'
let g:ale_c_clangd_options = ''
let g:ale_c_clangtidy_checks = []
let g:ale_c_clangtidy_executable = 'clang-tidy'
let g:ale_c_clangtidy_extra_options = ''
let g:ale_c_clangtidy_options = ''
let g:ale_c_cppcheck_executable = 'cppcheck'
let g:ale_c_cppcheck_options = '--enable=style'
let g:ale_c_cquery_cache_directory = '/home/chris/.cache/cquery'
let g:ale_c_cquery_executable = 'cquery'
let g:ale_c_flawfinder_error_severity = 6
let g:ale_c_flawfinder_executable = 'flawfinder'
let g:ale_c_flawfinder_minlevel = 1
let g:ale_c_flawfinder_options = ''
let g:ale_c_gcc_executable = 'gcc'
let g:ale_c_gcc_options = '-std=c11 -Wall'
let g:ale_c_parse_compile_commands = 0
let g:ale_c_parse_makefile = 0
Global Variables:
let g:ale_cache_executable_check_failures = v:null
let g:ale_change_sign_column_color = 0
let g:ale_command_wrapper = ''
let g:ale_completion_delay = v:null
let g:ale_completion_enabled = 0
let g:ale_completion_max_suggestions = v:null
let g:ale_echo_cursor = 1
let g:ale_echo_msg_error_str = 'Error'
let g:ale_echo_msg_format = '%code: %%s'
let g:ale_echo_msg_info_str = 'Info'
let g:ale_echo_msg_warning_str = 'Warning'
let g:ale_enabled = 1
let g:ale_fix_on_save = 0
let g:ale_fixers = {}
let g:ale_history_enabled = 1
let g:ale_history_log_output = 1
let g:ale_keep_list_window_open = 0
let g:ale_lint_delay = 200
let g:ale_lint_on_enter = 1
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_insert_leave = 1
let g:ale_lint_on_save = 1
let g:ale_lint_on_text_changed = 'normal'
let g:ale_linter_aliases = {}
let g:ale_linters = {}
let g:ale_linters_explicit = 0
let g:ale_list_vertical = 0
let g:ale_list_window_size = 10
let g:ale_loclist_msg_format = '%code: %%s'
let g:ale_lsp_root = {}
let g:ale_max_buffer_history_size = 20
let g:ale_max_signs = -1
let g:ale_maximum_file_size = v:null
let g:ale_open_list = 0
let g:ale_pattern_options = v:null
let g:ale_pattern_options_enabled = v:null
let g:ale_set_balloons = 0
let g:ale_set_highlights = 1
let g:ale_set_loclist = 1
let g:ale_set_quickfix = 0
let g:ale_set_signs = 1
let g:ale_sign_column_always = 0
let g:ale_sign_error = '>>'
let g:ale_sign_info = '--'
let g:ale_sign_offset = 1000000
let g:ale_sign_style_error = '>>'
let g:ale_sign_style_warning = '--'
let g:ale_sign_warning = '--'
let g:ale_sign_highlight_linenrs = 0
let g:ale_statusline_format = v:null
let g:ale_type_map = {}
let g:ale_use_global_executables = v:null
let g:ale_virtualtext_cursor = 0
let g:ale_warn_about_trailing_blank_lines = 1
let g:ale_warn_about_trailing_whitespace = 1
Command History:
(executable check - failure) cquery
(executable check - failure) flawfinder
(finished - exit code 1) ['/bin/zsh', '-c', '''gcc'' -S -x c -o /dev/null -iquote ''/home/chris'' -std=c11 -Wall - < ''/tmp/nvim4ptF35/16/t.c''']
<<>>
: In function ‘main’:
:7:5: error: ‘invalid’ undeclared (first use in this function)
:7:5: note: each undeclared identifier is reported only once for each function it appears in
:7:12: error: expected ‘;’ before ‘return’
<<>>
(finished - exit code 1) ['/bin/zsh', '-c', '''clang'' -S -x c -fsyntax-only -iquote ''/home/chris'' -std=c11 -Wall - < ''/tmp/nvim4ptF35/17/t.c''']
<<>>
:7:5: error: use of undeclared identifier 'invalid'
invalid
^
1 error generated.
<<>>
(finished - exit code 0) ['/bin/zsh', '-c', 'cd ''/home/chris'' && ''cppcheck'' -q --language=c --enable=style -I''/home/chris'' ''/tmp/nvim4ptF35/18/t.c''']
<<>>
(executable check - failure) cquery
(executable check - failure) flawfinder
(finished - exit code 1) ['/bin/zsh', '-c', '''gcc'' -S -x c -o /dev/null -iquote ''/home/chris'' -std=c11 -Wall - < ''/tmp/nvim4ptF35/19/t.c''']
<<>>
: In function ‘main’:
:7:5: error: ‘invalid’ undeclared (first use in this function)
:7:5: note: each undeclared identifier is reported only once for each function it appears in
:7:12: error: expected ‘;’ before ‘return’
<<>>
(finished - exit code 1) ['/bin/zsh', '-c', '''clang'' -S -x c -fsyntax-only -iquote ''/home/chris'' -std=c11 -Wall - < ''/tmp/nvim4ptF35/20/t.c''']
<<>>
:7:5: error: use of undeclared identifier 'invalid'
invalid
^
1 error generated.
<<>>
(finished - exit code 0) ['/bin/zsh', '-c', '''clang-tidy'' ''/home/chris/t.c'' -p ''/home/chris''']
<<>>
(finished - exit code 0) ['/bin/zsh', '-c', 'cd ''/home/chris'' && ''cppcheck'' -q --language=c --project=''compile_commands.json'' --enable=style ''/tmp/nvim4ptF
35/21/t.c''']
<<>>
(executable check - failure) cquery
(executable check - failure) flawfinder
(finished - exit code 1) ['/bin/zsh', '-c', '''gcc'' -S -x c -o /dev/null -iquote ''/home/chris'' -std=c11 -Wall - < ''/tmp/nvim4ptF35/22/t.c''']
<<>>
: In function ‘main’:
:7:5: error: ‘invalid’ undeclared (first use in this function)
:7:5: note: each undeclared identifier is reported only once for each function it appears in
:7:12: error: expected ‘;’ before ‘return’
<<>>
(finished - exit code 1) ['/bin/zsh', '-c', '''clang'' -S -x c -fsyntax-only -iquote ''/home/chris'' -std=c11 -Wall - < ''/tmp/nvim4ptF35/23/t.c''']
<<>>
:7:5: error: use of undeclared identifier 'invalid'
invalid
^
1 error generated.
<<>>
(finished - exit code 0) ['/bin/zsh', '-c', '''clang-tidy'' ''/home/chris/t.c'' -p ''/home/chris''']
<<>>
(finished - exit code 0) ['/bin/zsh', '-c', 'cd ''/home/chris'' && ''cppcheck'' -q --language=c --project=''compile_commands.json'' --enable=style ''/tmp/nvim4ptF
35/24/t.c''']
<<>>
(executable check - failure) cquery
(executable check - failure) flawfinder
(finished - exit code 1) ['/bin/zsh', '-c', '''gcc'' -S -x c -o /dev/null -iquote ''/home/chris'' -std=c11 -Wall - < ''/tmp/nvim4ptF35/25/t.c''']
<<>>
: In function ‘main’:
:7:5: error: ‘invalid’ undeclared (first use in this function)
:7:5: note: each undeclared identifier is reported only once for each function it appears in
:7:12: error: expected ‘;’ before ‘return’
<<>>
```