Rename PRIM_VMT_CALL as PRIM_VIRTUAL_METHOD_CALL.

This renaming aligns the name of the virtual method call primitive with its human-readable
description in primitive.cpp.

[test built and smoke-tested with checkChplInstall]
[reviewed by noakes]


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23700 3a8e244f-b0f2-452b-bcba-4c88e055c3ca