VS: Accept and translate '-T version=' values with three components

The VS 16.8 and VS 16.9 toolset versions differ only in their third
component.  The `vcvarsall` option `-vcvars_ver=` accepts a three
component version, so accept this format for VS toolset selection too.

Issue: #21922
43 jobs for !5903 with vs-toolset-version in 67 minutes and 52 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
manual #5604209
cmake linux docker build manual
build:centos6-release
passed #5604206
linux docker build cmake
build:debian10-iwyu

00:03:30

manual #5604210
linux-3.17 linux docker build cmake manual
build:fedora33-ninja
passed #5604208
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:04:59

passed #5604207
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:07:57

manual #5604212
nonconcurrent xcode-12.3 shell macos cmake manual
build:macos-makefiles
manual #5604211
nonconcurrent xcode-12.3 shell macos cmake manual
build:macos-ninja
passed #5604213
nonconcurrent msvc-19.28 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja

00:06:01

 
  Test
created #5604214
x11 linux-3.17 linux docker cmake
test:fedora33-ninja
created #5604216
nonconcurrent xcode-12.3 shell macos cmake
test:macos-makefiles
created #5604215
nonconcurrent xcode-12.3 shell macos cmake
test:macos-ninja
passed #5604217
windows cmake nonconcurrent msvc-19.28 vs2019 shell
test:windows-vs2019-x64-ninja

00:12:42

 
  Test Ext
created #5604220
linux docker cuda-rt cmake
test:cuda10.2-nvidia
created #5604218
linux-3.17 linux docker build cmake
test:debian10-ninja
created #5604219
linux-3.17 linux docker build cmake
test:fedora33-makefiles
created #5604221
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi
created #5604222
concurrent xcode-12.3 shell macos cmake
test:macos-xcode
passed #5604223
concurrent msvc-19.28 vs2019 shell windows cmake
test:windows-vs2019-x64

00:41:14

 
  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 ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager