Updated to find source files using VPATH. This makes writing build rules

much cleaner and easier.
Labeled .td as a suffix for tblgen files in Makefile.rules.
Modified build rules so that source files generated during the build are placed
in the build directory and not the source directory (and not in a Debug
directory).  This makes the system cleaner and allows us to have a read-only
source tree.

llvm-svn: 8424