[lit] Implement 'env' in the internal shell

The MSys 2 version of 'env' cannot be used to set 'TZ' in the
environment due to some portability hacks in the process spawning
compatibility layer[1]. This affects test/Object/archive-toc.test, which
tries to set TZ in the environment.

Other than that, this saves a subprocess invocation of a small unix
utility, which is makes the tests faster.

The internal shell does not support shell variable expansion, so this
idiom in the ASan tests isn't supported yet:
  RUN: env ASAN_OPTIONS=$ASAN_OPTIONS:opt=1 ...

[1] https://github.com/Alexpux/MSYS2-packages/issues/294

Differential Revision: http://reviews.llvm.org/D11350

llvm-svn: 242696