compiler: add debugger-callable AST dump functins
compiler: add debugger-callable AST dump functins

Introduce a set debug_go_* global functions that can be used to emit
AST dumps for Go statements and expressions from within GDB (for use
by people developing gccgo).

Change-Id: If4d9354f0400766eed0cc8449fb431523f1397cf
Reviewed-on: https://go-review.googlesource.com/c/162903
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend