debuginfo: Allow to activate GDB pretty printers in autotests.
debuginfo: Add GDB pretty printers for structs and enums.
debuginfo: Emit different autotest debugger scripts depending on GDB version.
debuginfo: Improve GDB version handling in compiletest tool
auto merge of #16322 : michaelwoerister/rust/gdb-pretty, r=alexcrichton

Also extends the autotest framework to let a test case choose if pretty printing should be enabled.