[LIT] Make util.executeCommand python3 friendly

Summary: The parameter `input` to `subprocess.Popen.communicate(...)` must be an object of type `bytes` . This is strictly enforced in python3. This patch (1) allows `to_bytes` to be safely called redundantly. (2) Explicitly convert `input` within `executeCommand`. This allows for usages like `executeCommand(['clang++', '-'], input='int main() {}\n')`.

Reviewers: ddunbar, BinaryKhaos, modocache, dim, EricWF

Reviewed By: EricWF

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28736

llvm-svn: 292308