Fix #7138: Suspend callers of macros compiled in the same run

[odersky]

Allows macros to be defined and called in the same project. The idea is to suspend
the callers of a macro (and their upstream dependencies), compile the sourcefile(s)
containing the macro(s), and then compile the suspended files in a second run.

The scheme detects and flags as errors calls to macros in the same file and mutual
dependencies between different files which would not work with suspensions.

It needs to be complemented by another mechanism that checks that class files loaded
by a macro class loader do not have associated source files that are currently compiled.

[nicolasstucki]

I tried converting all pos-macros and run-macros be compiled in a single step and use suspension. They all passed the tests. I will not add them to avoid duplication in tests.

[nicolasstucki]

@liufengyun the commits from @odersky LGTM but you should have a second look.

[liufengyun]

LGTM

While the utility enabled by the PR is doubtless, this PR makes it easier to write code that has subtle dependencies than before. The current error messages may not be helpful enough to resolve cyclic compilation errors, which may hurt usability.

What about putting the feature under a compiler flag?

[nicolasstucki]

@liufengyun the incremental compilation example actually works (or fails as expected). The incremental compiler trigger recompilation of macros when b is recompiled which makes it fail the same way it would from a clean compilation. I added it as a regression test.

[liufengyun]

rebased to fix conflicts