[generator] Fix MSBuild warning/error format for VS.

[jpobst]

In #715 we made generator warnings/errors give line/col information in more places, however the existing method for formatting the line/col 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/col information, VS parses the colon in C:\ instead, resulting in:

This is actually worse than what we previously had, as double-clicking it does nothing, since C is not a valid file on disk.

This commit rewrites the Format function to be a little clearer to read and adds the needed colon.