Don't explicitly use $SourceDir to find the tblgen files. This causes make

some confusion when trying to generate files (it probably couldn't tell that
./file and $(SourceDir)/file may be the same file).
Now, just let VPATH find everything, and list the primary tblgen file first in
the list of dependencies so that we can just use $< to reference it in the make
rule.

This should hopefully fix the nightly tester.

llvm-svn: 8433