RFR 8036942: Fix handling of multi-catch within a lambda

[Robert Field]

Please review compiler (javac) fix for:

https://bugs.openjdk.java.net/browse/JDK-8036942

Webrev:

http://cr.openjdk.java.net/~rfield/8036942/ 
<http://cr.openjdk.java.net/%7Erfield/8036942/>

The correct tree structure was not being created for a local variable of 
union type moved into a lambda method.  The full (un-erased) type needed 
to be passed to tree construction and tree construction needed to handle 
the of union types.

Thanks,
Robert