8364751: ConstantBootstraps.explicitCast contradictory specification for null-to-primitive

[liach]

ConstantBootstraps.explicitCast behaves like a snippet of code in its specification. However, in the rest of the nominal spec, it incorrectly assumes a null value and a primitive dstType results in a ClassCastException instead of the zero value of that primitive type. This is inconsistent with that snippet and the actual code behavior.

The specification is fixed, the test for explicitCast is merged into the main ConstantBootstraps test, and a new unit test case for value = null and dstType = char.class is added, verifying the outcome is 'u0000'.

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Issues

• JDK-8364751: ConstantBootstraps.explicitCast contradictory specification for null-to-primitive (Bug - P4)
• JDK-8365193: ConstantBootstraps.explicitCast contradictory specification for null-to-primitive (CSR)

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