Add switch to disable merge validation to support certain test scenarios (#936)

* Add switch to disable merge validation to support certain test scenarios

* made condition explicitly check 'true'

Author: Scottj1s

nuget/Microsoft.Windows.CppWinRT.targets

@@ -561,7 +561,8 @@ $(XamlMetaDataProviderPch)
         </PropertyGroup>
         <PropertyGroup>
             <!-- mdmerge.exe wants the folders to not have a trailing \ -->
-            <_MdMergeParameters>-v @(CppWinRTMdMergeMetadataDirectories->'-metadata_dir &quot;%(RelativeDir).&quot;', '&#x0d;&#x0a;')</_MdMergeParameters>
+            <_MdMergeParameters Condition="'$(CppWinRTMergeNoValidate)'!='true'">-v</_MdMergeParameters>
+            <_MdMergeParameters>$(_MdMergeParameters) @(CppWinRTMdMergeMetadataDirectories->'-metadata_dir &quot;%(RelativeDir).&quot;', '&#x0d;&#x0a;')</_MdMergeParameters>
             <_MdMergeParameters>$(_MdMergeParameters) @(CppWinRTMdMergeInputs->'-i &quot;%(Identity)&quot;', '&#x0d;&#x0a;')</_MdMergeParameters>
             <_MdMergeParameters>$(_MdMergeParameters) -o &quot;$(CppWinRTMergedDir.TrimEnd('\'))&quot; -partial $(_MdMergeDepth)</_MdMergeParameters>
         </PropertyGroup>

nuget/readme.md

@@ -61,6 +61,7 @@ C++/WinRT behavior can be customized with these project properties:
 | CppWinRTVerbosity | low \| *normal \| high | Sets the [importance](https://docs.microsoft.com/en-us/visualstudio/msbuild/message-task?view=vs-2017) of C++/WinRT build messages (see below) |
 | CppWinRTNamespaceMergeDepth | *1 | Sets the depth of namespace merging (Xaml apps require 1) |
 | CppWinRTRootNamespaceAutoMerge | true \| *false | Sets the namespace merge depth to be the length of the root namespace |
+| CppWinRTMergeNoValidate | true \| *false | Disables mdmerge validation |
 | CppWinRTUsePrefixes | *true \| false | Uses a dotted prefix namespace convention (versus a nested folder convention) |
 | CppWinRTPath | ...\cppwinrt.exe | NuGet package-relative path to cppwinrt.exe, for custom build rule invocation |
 | CppWinRTParameters | "" | Custom cppwinrt.exe command-line parameters (be sure to append to existing) |