Removed Diagnostics from sys.ts in order to avoid cyclical build dependency.

Specifically, processDiagnosticMessages.ts was dependent on sys.ts, which was dependent on the rest of the compiler,
which meant that in a broken state of diagnostics, you could never compile processDiagnosticMessages.ts.
Addressed CR feedback.
Merge pull request #932 from Microsoft/breakCycleInDiagnosticGeneration

Removed Diagnostics from sys.ts in order to avoid cyclical build dependency