VS: Generalize CSharp /langversion flag mapping

Map the `/langversion:` flag followed by any value to a `LangVersion`
element with the specified value in the `.csproj` file.

Fixes: #22089
30 jobs for !6031 with vs-csharp-langversion in 22 minutes and 5 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
manual #5721659
linux docker build cmake manual
build:centos6-x86_64
passed #5721656
linux docker build cmake
build:debian10-iwyu

00:04:23

manual #5721660
linux-3.17 linux docker build cmake manual
build:fedora33-ninja
passed #5721658
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:05:02

passed #5721657
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:06:44

manual #5721662
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-makefiles
manual #5721661
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-ninja
passed #5721663
nonconcurrent msvc-19.28-16.9 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja

00:02:57

 
  Test
created #5721664
x11 linux-3.17 linux docker cmake
test:fedora33-ninja
created #5721666
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-makefiles
created #5721665
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-ninja
passed #5721667
nonconcurrent msvc-19.28-16.9 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja

00:05:41

 
  Test Ext
created #5721670
linux docker cuda-rt cmake
test:cuda10.2-nvidia
created #5721668
linux-3.17 linux docker build cmake
test:debian10-ninja
created #5721669
linux-3.17 linux docker build cmake
test:fedora33-makefiles
created #5721671
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi
created #5721672
concurrent xcode-12.4 shell macos cmake
test:macos-x86_64-xcode
failed #5721673
concurrent msvc-19.28-16.9 vs2019 shell windows cmake
test:windows-vs2019-x64

00:10:56

 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-stager

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

 
Name Stage Failure
failed
test:windows-vs2019-x64 Test Ext
   SubmitURL: http://open.cdash.org/submit.php?project=CMake
Uploaded: C:/glr/builds/cmake/cmake ci ext/0/build/Testing/20210421-1446/Done.xml
Submission successful
CMake Error at C:/glr/builds/cmake/cmake ci ext/0/.gitlab/ci/ctest_test_external.cmake:84 (message):
Failed to test


Cleaning up file based variables
ERROR: Job failed: exit status 1