Rollup merge of rust-lang#39983 - AndrewGaspar:rust-windbg, r=brson

Adds rust-windbg.cmd script

Adds rust-gdb/rust-lldb equivalent for windbg that loads the Rust .natvis files on start.

This change modifies the bootstrap code to add rust-windbg to bin and the .natvis files to lib/rustlib/etc.

Example usage from cmd or PowerShell:
```
rust-windbg -c "bu rs_f442289d74765418!rs::main;g" target\debug\rs.exe
```

src/bootstrap/dist.rs

@@ -254,7 +254,12 @@ pub fn debugger_scripts(build: &Build,
         install(&build.src.join("src/etc/").join(file), &dst, 0o644);
     };
     if host.contains("windows-msvc") {
-        // no debugger scripts
+        // windbg debugger scripts
+        install(&build.src.join("src/etc/rust-windbg.cmd"), &sysroot.join("bin"),
+            0o755);
+
+        cp_debugger_script("natvis/libcore.natvis");
+        cp_debugger_script("natvis/libcollections.natvis");
     } else {
         cp_debugger_script("debugger_pretty_printers_common.py");
 

src/etc/rust-windbg.cmd

@@ -0,0 +1,18 @@
+@echo off
+setlocal
+
+REM Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+REM file at the top-level directory of this distribution and at
+REM http://rust-lang.org/COPYRIGHT.
+REM
+REM Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+REM http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+REM <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+REM option. This file may not be copied, modified, or distributed
+REM except according to those terms.
+
+for /f "delims=" %%i in ('rustc --print=sysroot') do set rustc_sysroot=%%i
+
+set rust_etc=%rustc_sysroot%\lib\rustlib\etc
+
+windbg -c ".nvload %rust_etc%\libcore.natvis;.nvload %rust_etc%\libcollections.natvis;" %*