Repository: rust
Issue: Compiler selects invalid `--lldb-python` path
Labels: A-testsuite,A-debuginfo,T-compiler,T-bootstrap,C-bug,A-compiletest

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For my mac (macOS Mojave), the `--lldb-python` path is being set to `/usr/bin/python3`, which doesn't exist.
Furthermore, `/usr/bin` is immutable, so I cannot simply symlink to `/usr/local/bin/python3`, the result of `which python3`.

This is causing the `debuginfo` test suite to fail.

### Code

```
./x.py -i test src/test/debuginfo
```


### Meta
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If you're using the stable version of the compiler, you should also check if the
bug also exists in the beta or nightly versions.
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rust version:
Top of `master`, specifically:
https://github.com/rust-lang/rust/commit/3dbade652ed8ebac70f903e01f51cd92c4e4302c

### Error output

```
running 115 tests
iFFFFFFFiFFFFFiFFFFFFFFFFFiFFFFiFFiiFFiFFiFFiiFiFFFFFFFFFFFFFiFiFiiFFFFFFFFiFFFFFiFiiFFFF.iFFFFiFFiF 100/115
FFFFiiFiFFFFFFF
failures:

---- [debuginfo-lldb] debuginfo/basic-types-globals-metadata.rs stdout ----
NOTE: compiletest thinks it is using LLDB version 1100
NOTE: compiletest thinks it is using LLDB without native rust support

error: Failed to setup Python process for LLDB script: No such file or directory (os error 2)
[ERROR compiletest::runtest] fatal error, panic: "Failed to setup Python process for LLDB script: No such file or directory (os error 2)"
thread 'main' panicked at 'fatal error', src/tools/compiletest/src/runtest.rs:2133:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
// …
```

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Include a backtrace in the code block by setting `RUST_BACKTRACE=1` in your
environment. E.g. `RUST_BACKTRACE=1 cargo build`.
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<details><summary><strong>Backtrace</strong></summary>
<p>

```
running 115 tests
iFFFFFFFiFFFFFiFFFFFFFFFFFiFFFFiFFiiFFiFFiFFiiFiFFFFFFFFFFFFFiFiFiiFFFFFFFFiFFFFFiFiiFFFF.iFFFFiFFiF 100/115
FFFFiiFiFFFFFFF
failures:

---- [debuginfo-lldb] debuginfo/basic-types-globals-metadata.rs stdout ----
NOTE: compiletest thinks it is using LLDB version 1100
NOTE: compiletest thinks it is using LLDB without native rust support

error: Failed to setup Python process for LLDB script: No such file or directory (os error 2)
[ERROR compiletest::runtest] fatal error, panic: "Failed to setup Python process for LLDB script: No such file or directory (os error 2)"
thread 'main' panicked at 'fatal error', src/tools/compiletest/src/runtest.rs:2133:9
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::io::impls::<impl std::io::Write for alloc::boxed::Box<W>>::write_fmt
   4: std::sys_common::backtrace::print
   5: std::panicking::default_hook::{{closure}}
   6: std::panicking::default_hook
   7: std::panicking::rust_panic_with_hook
   8: std::panicking::begin_panic
   9: compiletest::runtest::TestCx::fatal
  10: compiletest::runtest::TestCx::cmd2procres
  11: compiletest::runtest::TestCx::run_revision
  12: compiletest::runtest::run
  13: core::ops::function::FnOnce::call_once{{vtable.shim}}
  14: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  15: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  16: __rust_maybe_catch_panic
  17: std::panicking::try
  18: test::run_test_in_process
  19: test::run_test::run_test_inner
  20: test::run_test
  21: test::run_tests
  22: test::console::run_tests_console
  23: compiletest::main
  24: std::rt::lang_start::{{closure}}
  25: std::panicking::try::do_call
  26: __rust_maybe_catch_panic
  27: std::panicking::try
  28: std::rt::lang_start_internal
  29: main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
```

</p>
</details>