Lit: Allow overriding llvm tool paths+arguments, make -D an alias for --param

These changes allow usages where you want to pass an additional
commandline option to all invocations of a specific llvm tool. Example:

> llvm-lit -Dllc=llc -enable-misched -verify-machineinstrs

Differential Revision: http://reviews.llvm.org/D9487

llvm-svn: 236461