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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8433 91177308-0d34-0410-b5e6-96231b3b80d8