[dotest] Don't try to guess the llvm binary dir.

Now that all supported build systems create a valid dotest.py
invocation, we no longer need to guess the directory where any of the
llvm tools live. Additionally, the current logic is incomplete: it
doesn't try to find any other tools than FileCheck, such as dsymutil for
example.

If no FileCheck is provided, we should print a warning and skip the
tests that need it, but that's not part of this patch.

Differential revision: https://reviews.llvm.org/D66893

llvm-svn: 370232