[ScopInfo] Rename ScopStmt::contains(BB) to represents(BB). NFC.

In future, there will be no more a 1:1 correspondence between statements
and basic blocks, the name `contains` does not correctly capture their
relationship. A BB may infact comprise of multiple statements; hence we
describe a statement 'representing' a basic block.

Differential Revision: https://reviews.llvm.org/D35838

llvm-svn: 308982