[generator] Fix MSBuild warning/error format for Visual Studio (#765)

In b858dc5 we updated `generator` warnings/errors to give line &
column information in more places. However, the existing method for
formatting the line & column information was wrong:
// Correct
C:\code\Metadata.xml(2, 6): warning BG8A04: <attr path="/api/package[@name='androidx.appcompat.widget']/class[@name='RoundRectDrawableWithShadow']"/> matched no nodes.
// Incorrect
C:\code\Metadata.xml(2, 6) warning BG8A04: <attr path="/api/package[@name='androidx.appcompat.widget']/class[@name='RoundRectDrawableWithShadow']"/> matched no nodes.
By omitting the colon after the line & column information,
Visual Studio parses the colon within `C:\` instead, resulting in:
![image](https://user-images.githubusercontent.com/179295/102400384-7500b100-3fa7-11eb-8f35-aae2a04f3a58.png)
* Filename: `C`
* Line number: 1
This is actually worse than what we previously had, as double-clicking
it does nothing, as `C` is not a valid file on disk.
Rewrite the `Report.Format()` method to be a little clearer to read
and add the required colon.

Author: jpobst

tests/generator-Tests/Unit-Tests/ReportTests.cs

@@ -0,0 +1,30 @@
+usingSystem;
+usingSystem.Collections.Generic;
+usingSystem.Linq;
+usingSystem.Text;
+usingSystem.Threading.Tasks;
+usingMonoDroid.Generation;
+usingNUnit.Framework;
+
+namespacegeneratortests
+{
+publicclassReportTests
+{
+[Test]
+publicvoidFormatTests()
+{
+varcode=0x37;
+varmsg="There was a {0} error";
+varargs="bad";
+varsourcefile=@"C:\code\test.cs";
+varline=32;
+varcol=12;
+
+Assert.AreEqual("error BG0037: There was a bad error",Report.Format(true,code,null,0,0,msg,args));
+Assert.AreEqual(@"C:\code\test.cs: error BG0037: There was a bad error",Report.Format(true,code,sourcefile,0,0,msg,args));
+Assert.AreEqual(@"C:\code\test.cs(32): error BG0037: There was a bad error",Report.Format(true,code,sourcefile,line,0,msg,args));
+Assert.AreEqual(@"C:\code\test.cs(32, 12): error BG0037: There was a bad error",Report.Format(true,code,sourcefile,line,col,msg,args));
+Assert.AreEqual(@"C:\code\test.cs(32, 12): warning BG0037: There was a bad error",Report.Format(false,code,sourcefile,line,col,msg,args));
+}
+}
+}

tools/generator/Utilities/Report.cs

@@ -129,9 +129,25 @@ public static string FormatCodedMessage (bool error, LocalizedMessage message, p
 
 publicstaticstringFormat(boolerror,interrorCode,stringsourceFile,intline,intcolumn,stringformat,paramsobject[]args)
 {
-varorigin=sourceFile!=null?sourceFile+(line>0?column>0?$"({line}, {column})":$"({line})":null)+' ':null;
-returnstring.Format("{0}{1} BG{2:X04}: ",origin,error?"error":"warning",errorCode)+
-string.Format(format,args);
+varorigin=FormatOrigin(sourceFile,line,column);
+
+return$"{origin}{(error?"error":"warning")} BG{errorCode:X04}: "+string.Format(format,args);
+}
+
+staticstringFormatOrigin(stringsourceFile,intline,intcolumn)
+{
+if(string.IsNullOrWhiteSpace(sourceFile))
+returnnull;
+
+varret=sourceFile;
+
+if(line==0)
+returnret+": ";
+
+if(column>0)
+returnret+$"({line}, {column}): ";
+
+returnret+$"({line}): ";
 }
 }