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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328072 91177308-0d34-0410-b5e6-96231b3b80d8