[lit] Modify LIT to accept environment variable LIT_FILTER to select tests.

This is especially useful when lit is invoked indirectly by the build
system, and additional arguments can not be easily specified.

Differential Revision: https://reviews.llvm.org/D35091

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307339 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed