RFR 8172443: Change use of tree.pos to line:col in rawDiagnostics

[Maurizio Cimadamore]

Hi,
this is a fix to our raw diagnostic machinery that will allow for more 
robust compiler negative tests. Currently, when a functional expression 
is found in a diagnostic, it can sometimes be represented using the 
absoluted cursor position of the expression in the source file. This 
makes the golden files for such tests extremely unreliable, as simply 
adding/removing chars from lines before the functional expression would 
result in golden file mismatches. This is extremely annoying when e.g. 
the jtreg header of a test must be change for some reason (e.g. to add 
an extra bug id).

The solution is to use a more robust encoding with line:col - so that 
there's less dependencies on what happens on previous lines.

http://cr.openjdk.java.net/~mcimadamore/8172443/

Cheers
Maurizio