JEP 334 Constants API pr tracking

[theresa-m]

Prerequisites (merge in order):

• Make Java 12 compile: Fix Java 12 build failures #3980 (issues Recent Java12 changes have moved some classes causing builds to now fail #3768 Java 12 extensions update causing build failure #4158)
• Stubs for JEP: Stubs for Constants API (jep334) #4030
• Java 12 test compile: Java 12 test compilation #4531
• Java12AndUp test framework: Setup for Java12andUp tests (jep334) #4104 (moved to Methods and tests for jep334 String #4105 )

New methods for constants api:

• String methods: Methods and tests for jep334 String #4105
• Class methods: Methods and tests for jep334 Class #4157
• MethodType methods (depends on Class pr): Methods and tests for jep334 MethodType #4194
• MethodHandle methods (depends on Class and MethodType prs): Method and tests for jep334 MethodHandle #4412
• VarHandle and VarHandleDesc methods (depends on Class pr): Jep334 VarHandle and VarHandleDesc #4449

[theresa-m]

all prs have been merged