Add lowercase OS name feature
Add lowercase OS name feature

Summary:
Some tests already make use of OS feature names, e.g. 'linux' and 'freebsd',
but they are not actually currently set by lit.

Reviewers: pcc, eugenis

Reviewed By: eugenis

Subscribers: emaste, krytarowski, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D49115

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@336633 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt