Move Compiler.h from Support to Demangler to fix layering.

Support depends on Demangle (Support/Unix/Signals.inc), so Demangle
including Support/Compiler.h created a circular dependency.

Leave a forwarding shim of Compiler.h because it makes more sense for
users (a deeper fix might involve splitting Support into lower and upper
Support - but that also sounds a bit weird/awkward) than thinking about
the dependency on the Demangler.

llvm-svn: 328072