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 build:centos6-release #5604209
cmake linux docker build manual
passed build:debian10-iwyu #5604206
linux docker build cmake

00:03:30

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

00:04:59

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

00:07:57

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

00:06:01

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

00:12:42

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

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