[lit] Add a generic build script with a lit substitution.

This adds a script called build.py as well as a lit substitution
called %build that we can use to invoke it.  The idea is that
this allows a lit test to build test inferiors without having
to worry about architecture / platform specific differences,
command line syntax, finding / configurationg a proper toolchain,
and other issues.  They can simply write something like:

%build --arch=32 -o %t.exe %p/Inputs/foo.cpp

and it will just work.  This paves the way for being able to
run lit tests with multiple configurations, platforms, and
compilers with a single test.

Differential Revision: https://reviews.llvm.org/D54914

llvm-svn: 348058