Avoid using the fallback context for runtime files

For requests on the runtime files, only execute them in the
fallback context if there is no other context available for it.

Especially avoid indexing the runtime in the fallback context.
This is saving around 600mb of RAM.

For eng/ide/ada_language_server#1596

Author: Boulanger

source/ada/lsp-ada_handlers.adb

@@ -247,8 +247,12 @@ package body LSP.Ada_Handlers is
       --  Check if the given file belongs to the runtime.
 
       function Is_A_Source (Self : LSP.Ada_Contexts.Context) return Boolean is
-        (Is_Runtime_File or else Self.Is_Part_Of_Project (File));
+        ((Is_Runtime_File and then not Self.Is_Fallback_Context)
+         or else Self.Is_Part_Of_Project (File));
       --  Return True if File is a source of the project held by Context
+      --  Avoid considering runtime files as sources for the fallback context,
+      --  if there is no context available for this file then we will still
+      --  use the fallback later.
 
       Contexts : constant LSP.Ada_Context_Sets.Context_Lists.List :=
         (Self.Contexts.Each_Context (Is_A_Source'Unrestricted_Access));