RFR (XS): 8067728: Flag::unlock_diagnostic() should be called Flag::clear_diagnostic()

[Rachel Protacio]

Hi,

Please review this tiny change, renaming Flag::unlock_diagnostic() as 
Flag::clear_diagnostic.

Bug: https://bugs.openjdk.java.net/browse/JDK-8067728
Open webrev: http://cr.openjdk.java.net/~rprotacio/8067728.00/

Thanks!
Rachel