[PM] Rename lookupPass to lookUpPass.

Summary:
"Lookup" is a noun ("lookup table"), "look up" is a verb ("look up
'table' in the dictionary").

Reviewers: chandlerc

Subscribers: silvas, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D27374

llvm-svn: 288598