Remove /await flag for compat with C++20 (#756)

Author: kennykerr

nuget/Microsoft.Windows.CppWinRT.targets

@@ -828,7 +828,8 @@ $(XamlMetaDataProviderPch)
     <!--Append any additional item metadata after all default and project settings have been applied-->
     <ItemDefinitionGroup>
       <ClCompile>
-        <AdditionalOptions>%(AdditionalOptions) /bigobj /await</AdditionalOptions>
+        <AdditionalOptions>%(AdditionalOptions) /bigobj</AdditionalOptions>
+        <AdditionalOptions Condition="'%(ClCompile.LanguageStandard)' == 'stdcpp17'">%(AdditionalOptions) /await</AdditionalOptions>
         <AdditionalIncludeDirectories>%(AdditionalIncludeDirectories);$(GeneratedFilesDir)</AdditionalIncludeDirectories>
       </ClCompile>
       <Midl Condition="'$(CppWinRTModernIDL)' != 'false'">

nuget/readme.md

@@ -36,7 +36,7 @@ It sets the following project properties and item metadata:
 | XamlLanguage | CppWinRT | Directs the Xaml compiler to generate C++/WinRT code |
 | ClCompile.CompileAsWinRT | *false | Enables ISO C++ compilation (disables C++/CX) |
 | ClCompile.LanguageStandard | *stdcpp17 | Enables C++17 language features |
-| ClCompile.AdditionalOptions | /bigobj /await | Enables support for large object files and coroutines |
+| ClCompile.AdditionalOptions | /bigobj | Enables support for large object files |
 | ClCompile.AdditionalIncludeDirectories | GeneratedFilesDir | Adds $(GeneratedFilesDir) to the C++ include dirs |
 | Link.AdditionalDependencies | WindowsApp.lib | Umbrella library for Windows Runtime imports |
 | Midl.AdditionalOptions | /reference ... | Enables faster compilation with winmd references (versus idl imports) |