rustc: Enable writing "unsafe extern fn() {}"

Previously, the parser would not allow you to simultaneously implement a
function with a different abi as well as being unsafe at the same time. This
extends the parser to allow functions of the form:

    unsafe extern fn foo() {
        // ...
    }

The closure type grammar was also changed to reflect this reversal, types
previously written as "extern unsafe fn()" must now be written as
"unsafe extern fn()". The parser currently has a hack which allows the old
style, but this will go away once a snapshot has landed.

Closes #10025

[breaking-change]
debuginfo: Split debuginfo autotests into debuginfo-gdb and debuginfo-lldb
auto merge of #13726 : michaelwoerister/rust/lldb-autotests, r=alexcrichton

This pull request contains preparations for adding LLDB autotests:
+ the debuginfo tests are split into debuginfo-gdb and debuginfo-lldb
  + the `compiletest` tool is updated to support the debuginfo-lldb mode
  + tests.mk is modified to provide debuginfo-gdb and debuginfo-lldb make targets
  + GDB test cases are moved from `src/test/debug-info` to `src/test/debuginfo-gdb`
+ configure will now look for LLDB and set the appropriate CFG variables
+ the `lldb_batchmode.py` script is added to `src/etc`. It emulates GDB's batch mode

The LLDB autotests themselves are not part of this PR. Those will probable require some manual work on the test bots to make them work for the first time. Better to get these unproblematic preliminaries out of the way in a separate step.
auto merge of #14005 : alexcrichton/rust/extern-unsafe, r=pcwalton

Previously, the parser would not allow you to simultaneously implement a
function with a different abi as well as being unsafe at the same time. This
extends the parser to allow functions of the form:

    unsafe extern fn foo() {
        // ...
    }

The closure type grammar was also changed to reflect this reversal, types
previously written as "extern unsafe fn()" must now be written as
"unsafe extern fn()". The parser currently has a hack which allows the old
style, but this will go away once a snapshot has landed.

Closes #10025

[breaking-change]
io: Implement process wait timeouts

This implements set_timeout() for std::io::Process which will affect wait()
operations on the process. This follows the same pattern as the rest of the
timeouts emerging in std::io::net.

The implementation was super easy for everything except libnative on unix
(backwards from usual!), which required a good bit of signal handling. There's a
doc comment explaining the strategy in libnative. Internally, this also required
refactoring the "helper thread" implementation used by libnative to allow for an
extra helper thread (not just the timer).

This is a breaking change in terms of the io::Process API. It is now possible
for wait() to fail, and subsequently wait_with_output(). These two functions now
return IoResult<T> due to the fact that they can time out.

Additionally, the wait_with_output() function has moved from taking `&mut self`
to taking `self`. If a timeout occurs while waiting with output, the semantics
are undesirable in almost all cases if attempting to re-wait on the process.
Equivalent functionality can still be achieved by dealing with the output
handles manually.

[breaking-change]

cc #13523