move InfoString from ErrorReporting to Decorators and

remove all isSensical/SuppressedMessage logic from InfoString
remove var ContextBase.suppressNonSensicalErrors, and

decouple Diagnostic from ContextBase
add d string interpolator (marks nonsensical error messages)
remove SuppressedMessage exception and update Diagnostic as needed
replace i"..." by d"..." if string might go into a

Diagnostic and might be nonsensical

Conflicts:
	src/dotty/tools/dotc/typer/Checking.scala
	src/dotty/tools/dotc/typer/TypeAssigner.scala
replace some s"..." by i"..."

Conflicts:
	src/dotty/tools/dotc/typer/TypeAssigner.scala
fix: isSensical in d"..." impl must inspect contents of List arg
fix: do not count suppressed messages for summary
Merge pull request #99 from samuelgruetter/infostring2

Error reporting improvements