RFR: 8215368: Make Check.checkOverride call diagnosticPositionFor lazily

[Liam Miller-Cushon]

Hi,

Please consider this trivial performance fix to Check.checkOverride, which
achieved roughly a 2x speedup on a compilation of a pathological generated
source file.

bug: https://bugs.openjdk.java.net/browse/JDK-8215368
webrev: http://cr.openjdk.java.net/~cushon/8215368/webrev.00
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