Fix the dbg config on Windows

clang-cl has a different syntax for specifying the C++ standard
version so was rejecting the fix added in #1184 for the DCHECK in V8's
graph-visualizer.cc that needs C++20. V8 DCHECKs are enabled for dbg
builds. This is fixed in .bazelrc.

To add to my own confusion when looking into this issue, the junction
we create in the workspace folder, `external`, was stable. The
junction is created to make it easy to find sources to downloaded
dependencies and also to help VSCode find the right sources when
bazel build emits compilation errors. Unfortunately, the junction
was stale locally so following the error links reported in VSCode went
to an old version of V8 on the disk with entirely different code that
could not possibly have caused the error observed. As a result, the
script that creates the junction when VSCode opens the project is
updated to delete any existing `external` junction and create it anew.

.bazelrc

@@ -13,7 +13,8 @@ build --host_per_file_copt='external/dawn@-Wno-shorten-64-to-32,-Wno-unneeded-in
 
 # Enable C++20 to support std::unordered_map::contains(). Not sure why this is needed here, V8
 # is supposed to work with C++14.
-build --per_file_copt='external/v8/src/compiler/graph-visualizer.cc@-std=c++20'
+build:unix --per_file_copt='external/v8/src/compiler/graph-visualizer.cc@-std=c++20'
+build:windows --per_file_copt='external/v8/src/compiler/graph-visualizer.cc@/std:c++20'
 
 # Speed up sandboxed compilation, particularly on I/O-constrained and non-Linux systems
 # https://bazel.build/reference/command-line-reference#flag--reuse_sandbox_directories

tools/windows/create-external.bat

@@ -7,9 +7,12 @@ FOR /f %%i IN ('bazel info output_path') do SET "output_path=%%i"
 FOR /f %%i IN ('bazel info workspace') do SET "workspace=%%i"
 SET "external=%output_path%\..\..\..\external"
 
-IF NOT EXIST "%workspace%\external" (
-  MKLINK /J "%workspace%\external" "%external%"
-)
+@REM Delete the convenience junction external if it exists (it maybe stale if it does).
+rmdir  "%workspace%\external" 2>NUL
+
+@REM Create the convenience junction external anew for easy access to sources and for
+@REM VSCode to be able to open files at points of error when compilation fails.
+MKLINK /J "%workspace%\external" "%external%"
 
 SET "compile_commands=%workspace%\compile_commands.json"
 IF EXIST "%compile_commands%" (