Modified the Objective-C lexer and parser (only
in debugger mode) to accept @import declarations
and pass them to the debugger.

In the preprocessor, accept import declarations
if the debugger is enabled, but don't actually
load the module, just pass the import path on to
the preprocessor callbacks.

In the Objective-C parser, if it sees an import
declaration in statement context (usual for LLDB),
ignore it and return a NullStmt.