ICU-23054 Make the MSVC stdcpplatest build work as intended

Author: eggrobin

.github/workflows/icu4c.yml

@@ -376,10 +376,10 @@ jobs:
             build_flags: '/p:Configuration=Debug /p:Platform=Win32'
           - test_flags: 'arm Release'
             build_flags: '/p:Configuration=Release /p:Platform=ARM'
+          - test_flags: 'x64 Release cpplatest'
+            build_flags: '/p:OverrideLanguageStandard=stdcpplatest /p:Configuration=Release /p:Platform=x64'
           - test_flags: 'x64 Release'
-            build_flags: '/p:LanguageStandard=stdcpplatest /p:Configuration=Release /p:Platform=x64'
-          - test_flags: 'x64 Release'
-            build_flags: '/p:_HAS_EXCEPTIONS=0 /p:Configuration=Release /p:Platform=x64'
+            build_flags: '/p:Configuration=Release /p:Platform=x64'
     steps:
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
       - name: Set up MSBuild

icu4c/source/allinone/Build.Windows.ProjectConfiguration.props

@@ -97,7 +97,8 @@
       <!-- Enable parallel compilation for faster builds. -->
       <MultiProcessorCompilation>true</MultiProcessorCompilation>
       <!-- Set the C/C++ versions supported. -->
-      <LanguageStandard>stdcpp17</LanguageStandard>
+      <LanguageStandard Condition="'$(OverrideLanguageStandard)'==''">stdcpp17</LanguageStandard>
+      <LanguageStandard Condition="'$(OverrideLanguageStandard)'!=''">$(OverrideLanguageStandard)</LanguageStandard>
       <LanguageStandard_C>stdc11</LanguageStandard_C>
     </ClCompile>
     <ResourceCompile>

icu4c/source/allinone/Build.Windows.UWP.ProjectConfiguration.props

@@ -46,7 +46,8 @@
         U_PLATFORM_HAS_WINUWP_API=1;
       </PreprocessorDefinitions>
       <!-- Set the C/C++ versions supported. -->
-      <LanguageStandard>stdcpp17</LanguageStandard>
+      <LanguageStandard Condition="'$(OverrideLanguageStandard)'==''">stdcpp17</LanguageStandard>
+      <LanguageStandard Condition="'$(OverrideLanguageStandard)'!=''">$(OverrideLanguageStandard)</LanguageStandard>
       <LanguageStandard_C>stdc11</LanguageStandard_C>
     </ClCompile>
     <ResourceCompile>