[analyzer][UninitializedObjectChecker] Refactored checker options

Since I plan to add a number of new flags, it made sense to encapsulate
them in a new struct, in order not to pollute FindUninitializedFields's
constructor with new boolean options with super long names.

This revision practically reverts D50508, since FindUninitializedFields
now accesses the pedantic flag anyways.

Differential Revision: https://reviews.llvm.org/D51679

llvm-svn: 342219