RFR: JDK-8222744: add support for generating method handles from a variable symbol

[Vicente Romero]

Please review the fix for [1] at [2]. This is a small follow up after 
[3] which introduced a new internal constant pool API in javac. We 
realized that it was not possible to generate with the new API method 
handles with reference kinds:

  * REF_getField
  * REF_putField
  * REF_getStatic
  * REF_putStatic

This patch adds that feature,

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8222744
[2] http://cr.openjdk.java.net/~vromero/8222744/
[3] https://bugs.openjdk.java.net/browse/JDK-8222289