Split Disassembler.h in two to fix dependencies

Split Disassembler.h in two to fix dependencies

Support includes this header for the typedefs - but logically it's part
of the MC/Disassembler library that implements the functions. Split the
header so as not to create a circular dependency.

This is another case where probably inverting the llvm-c implementation
might be best (rather than core llvm libraries implementing the parts of
llvm-c - instead llvm-c could be its own library, depending on all the
parts of LLVM's core libraries to then implement llvm-c on top of
them... if that makes sense)

llvm-svn: 328744