[generator] Provide line/col numbers for api.xml warnings.

[jpobst]

Today, when you get a warning while trying to bind an api.xml file, no source information is provided:

BINDINGSGENERATOR : warning BG8700: Unknown return type 'com.blah.a.a.a.a' for member 
'Com.Blah.Foo.Sdk.LoggerGetUserLogger ()'. [C:\code\temp\Blah\Blah.csproj]

We can make the user experience a tiny bit nicer by adding the file name and line/col information in standard MSBuild format. This allows users to double click the error in VS and be taken directly to the correct location in the api.xml file:

BINDINGSGENERATOR : obj\Debug\api.xml(1684, 8) warning BG8700: Unknown return type 'com.blah.a.a.a.a' 
for member 'Com.Blah.Foo.Sdk.LoggerGetUserLogger ()'. [C:\code\temp\Blah\Blah.csproj]