Never inspect twice the same macro application

In Scala 2.10.4, this macro can produce a stack overflow :

    def foo(a: Any): Any = macro impl
    def impl(c: Context)(a: c.Expr[Any]): c.Expr[Any] = a

Here, an application such as `foo(someVal)` will produce the expansion
`someVal`. As expected, `someVal` has `original` tree `foo(someVal)`,
but if we inspect this tree, we will find that `someVal` has an
original tree, but it shouldn't.

Moreover, in Scala 2.11, some macros have their own application as
`original` trees.

See sbt/sbt#1237 for a description of these problems.

This commit fixes these two problems.

Fixes sbt/sbt#1237