[lit] Refactor out some more common lit configuration code.

[lit] Refactor out some more common lit configuration code.

debuginfo-tests has need to reuse a lot of common configuration
from clang and lld, and in general it seems like all of the
projects which are tightly coupled (e.g. lld, clang, llvm, lldb,
etc) can benefit from knowing about one other.  For example,
lldb needs to know various things about how to run clang in its
test suite.  Since there's a lot of common substitutions and
operations that need to be shared among projects, sinking this
up into LLVM makes sense.

In addition, this patch introduces a function add_tool_substitution
which handles all the dirty intricacies of matching tool names
which was previously copied around the various config files.  This
is now a simple straightforward interface which is hard to mess
up.

Differential Revision: https://reviews.llvm.org/D37944

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@313919 91177308-0d34-0410-b5e6-96231b3b80d8