VS: Suppress VS 16.9+ default /permissive flag

VS 16.9 changed its `ConformanceMode` setting from a `BoolProperty` to
an `EnumProperty`, and set it to `true` by default.  This causes the
`/permissive` flag to be passed to the compiler even if not requested by
the project.  In this case, add an explicit `ConformanceMode` default
setting to suppress the flag unless the project or user passes it.

Fixes: #21945
36 jobs for !5924 with vs-16.9-ConformanceMode in 34 minutes and 17 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build
manual #5624006
linux docker build cmake manual
build:centos6-release
passed #5624003
linux docker build cmake
build:debian10-iwyu

00:04:03

manual #5624007
linux-3.17 linux docker build cmake manual
build:fedora33-ninja
passed #5624005
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:05:08

passed #5624004
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:06:00

manual #5624009
nonconcurrent xcode-12.3 shell macos cmake manual
build:macos-makefiles
manual #5624008
nonconcurrent xcode-12.3 shell macos cmake manual
build:macos-ninja
passed #5624010
nonconcurrent msvc-19.28 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja

00:06:21

 
  Test
created #5624011
x11 linux-3.17 linux docker cmake
test:fedora33-ninja
created #5624013
nonconcurrent xcode-12.3 shell macos cmake
test:macos-makefiles
created #5624012
nonconcurrent xcode-12.3 shell macos cmake
test:macos-ninja
passed #5624014
nonconcurrent msvc-19.28 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja

00:15:41

 
  Test Ext
created #5624017
linux docker cuda-rt cmake
test:cuda10.2-nvidia
created #5624015
linux-3.17 linux docker build cmake
test:debian10-ninja
created #5624016
linux-3.17 linux docker build cmake
test:fedora33-makefiles
created #5624018
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi
created #5624019
cmake concurrent xcode-12.3 shell macos
test:macos-xcode
passed #5624020
concurrent msvc-19.28 vs2019 shell windows cmake
test:windows-vs2019-x64

00:11:06

 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager