[NFC][ScopBuilder] Move addRecordedAssumption to ScopBuilder

Scope of changes:
1) Moved addRecordedAssumptions to ScopBuilder.
2) Moved Assumption struct outside Scop class.
3) Refactored addRecordedAssumptions function. Replaced while loop by
for range loop.
4) Added function to clear processed Assumptions.

Differential Revision: https://reviews.llvm.org/D63572

llvm-svn: 366260