[Support] Force instantiation of isl dump() methods. NFC.

In order for debuggers to be able to call an inline method, it must have
been instantiated somewhere. The dump() methods are usually not used, so
add an instantiation in debug builds.

This allows to call .dump() on any isl++ object from the gcc/gdb and
Visual Studio debugger in debug builds with assertions enabled.
In optimized builds, even with assertions enabled, the dump() methods
are also inlined in GICHelper.cpp, so no externally visible symbols
will be available either.

Differential Revision: https://reviews.llvm.org/D38198

llvm-svn: 314395