Initialize hl-groups correctly if virtual text is loaded first (#3960)

If virtualtext.vim is autoloaded first, it will link
ALEVirtualTextWarning to ALEWarning. But ALEWarning is not initialized
yet, so it will create ALEWarning, but with no color definition set.

Shortly after, highlight.vim is autoloaded, which would usually link
ALEWarning to SpellCap, but only if ALEWarning is not already set.
However since ALEWarning is already initialized due to the previous
link, we skip this and never actually come around to properly
initializing it.

We fix this by initializing all highlight groups in highlight.vim, thus
satisfying the dependency of ALEVirtualTextWarning being initialized
after ALEWarning.

Fixes #3585

autoload/ale/highlight.vim

@@ -22,6 +22,26 @@ if !hlexists('ALEInfo')
     highlight link ALEInfo ALEWarning
 endif
 
+if !hlexists('ALEVirtualTextError')
+    highlight link ALEVirtualTextError ALEError
+endif
+
+if !hlexists('ALEVirtualTextStyleError')
+    highlight link ALEVirtualTextStyleError ALEVirtualTextError
+endif
+
+if !hlexists('ALEVirtualTextWarning')
+    highlight link ALEVirtualTextWarning ALEWarning
+endif
+
+if !hlexists('ALEVirtualTextStyleWarning')
+    highlight link ALEVirtualTextStyleWarning ALEVirtualTextWarning
+endif
+
+if !hlexists('ALEVirtualTextInfo')
+    highlight link ALEVirtualTextInfo ALEVirtualTextWarning
+endif
+
 " The maximum number of items for the second argument of matchaddpos()
 let s:MAX_POS_VALUES = 8
 let s:MAX_COL_SIZE = 1073741824 " pow(2, 30)

autoload/ale/virtualtext.vim

@@ -18,26 +18,6 @@ elseif has('textprop') && has('popupwin')
     let s:has_virt_text = 1
 endif
 
-if !hlexists('ALEVirtualTextError')
-    highlight link ALEVirtualTextError ALEError
-endif
-
-if !hlexists('ALEVirtualTextStyleError')
-    highlight link ALEVirtualTextStyleError ALEVirtualTextError
-endif
-
-if !hlexists('ALEVirtualTextWarning')
-    highlight link ALEVirtualTextWarning ALEWarning
-endif
-
-if !hlexists('ALEVirtualTextStyleWarning')
-    highlight link ALEVirtualTextStyleWarning ALEVirtualTextWarning
-endif
-
-if !hlexists('ALEVirtualTextInfo')
-    highlight link ALEVirtualTextInfo ALEVirtualTextWarning
-endif
-
 function! ale#virtualtext#Clear() abort
     if !s:has_virt_text
         return