RFR (S): JDK-8164086: Checked JNI pending exception check should be cleared when returning to Java frame

[David Simms]

Greetings,

JDK-8043224 Added warnings when using -Xcheck:jni when native code using 
the JNI API fails to check for exceptions. Returning to a Java frame 
should implicitly clear the need for checking for exceptions. Current 
JVM code does not, leading to a fair amount of false warnings from JDK 
core libraries.

Bug: https://bugs.openjdk.java.net/browse/JDK-8164086

Webrev: http://cr.openjdk.java.net/~dsimms/8164086/webrev0/

Testing: added a new jtreg test including some JNI use cases which 
should and should not produce warnings, passes on all platforms.

/David Simms