[AST] Add mechanism to notify module loading to other clients
This patch adds an optional callback function member to the AST Context.
The callback gets invoked when a new module (or overlay module) gets
loaded.
This can be used for instance by other clients, such as lldb, to perform
actions when a module gets loaded, like showing progress to the end-user.
rdar://94165195
Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>