RFR: 8245678: Avoid allocations in Executable.getAllGenericParameterTypes

[Claes Redestad]

Hi,

sponsoring this patch from Christoph Dreis.

Refactor Executable.getAllGenericParameterTypes to avoid allocations
in some cases.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8245678
Webrev: http://cr.openjdk.java.net/~redestad/8245678/open.00/

Testing: tier1+2

Original report: 
https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-May/066634.html

/Claes