Detect VS2017 using vswhere.

* Port of CoreFx change dotnet/corefx#22575

(cherry picked from commit 602e065)

run.cmd

@@ -16,19 +16,22 @@ setlocal
 :: is already configured to use that toolset. Otherwise, we will fallback to using the VS2015
 :: toolset if it is installed. Finally, we will fail the script if no supported VS instance
 :: can be found.
-if not defined VisualStudioVersion (
-  if defined VS150COMNTOOLS (
-    call "%VS150COMNTOOLS%\VsDevCmd.bat"
-    goto :Run
-  ) else if defined VS140COMNTOOLS (
-    call "%VS140COMNTOOLS%\VsDevCmd.bat"
-    goto :Run
-  )
+
+if defined VisualStudioVersion goto :Run
+
+set _VSWHERE="%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"
+if exist %_VSWHERE% (
+  for /f "usebackq tokens=*" %%i in (`%_VSWHERE% -latest -property installationPath`) do set _VSCOMNTOOLS=%%i\Common7\Tools
+)
+if not exist "%_VSCOMNTOOLS%" set _VSCOMNTOOLS=%VS140COMNTOOLS%
+if not exist "%_VSCOMNTOOLS%" (
   echo Error: Visual Studio 2015 or 2017 required.
   echo        Please see https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/developer-guide.md for build instructions.
   exit /b 1
 )
 
+call "%_VSCOMNTOOLS%\VsDevCmd.bat"
+
 :Run
 :: Clear the 'Platform' env variable for this session, as it's a per-project setting within the build, and
 :: misleading value (such as 'MCD' in HP PCs) may lead to build breakage (issue: #69).
@@ -55,4 +58,4 @@ pushd %~dp0
 call %_dotnet% %_toolRuntime%\run.exe "%_json%" %*
 popd
 
-exit /b %ERRORLEVEL%
+exit /b %ERRORLEVEL%