[analyzer] Restrict AnalyzerOptions' interface so that non-checker objects have to be registered

One of the reasons why AnalyzerOptions is so chaotic is that options can be
retrieved from the command line whenever and wherever. This allowed for some
options to be forgotten for a looooooong time. Have you ever heard of
"region-store-small-struct-limit"? In order to prevent this in the future, I'm
proposing to restrict AnalyzerOptions' interface so that only checker options
can be retrieved without special getters. I would like to make every option be
accessible only through a getter, but checkers from plugins are a thing, so I'll
have to figure something out for that.

This also forces developers who'd like to add a new option to register it
properly in the .def file.

This is done by

* making the third checker pointer parameter non-optional, and checked by an
  assert to be non-null.
* I added new, but private non-checkers option initializers, meant only for
  internal use,
* Renamed these methods accordingly (mind the consistent name for once with
  getBooleanOption!):
  - getOptionAsString -> getCheckerStringOption,
  - getOptionAsInteger -> getCheckerIntegerOption
* The 3 functions meant for initializing data members (with the not very
  descriptive getBooleanOption, getOptionAsString and getOptionAsUInt names)
  were renamed to be overloads of the getAndInitOption function name.
* All options were in some way retrieved via getCheckerOption. I removed it, and
  moved the logic to getStringOption and getCheckerStringOption. This did cause
  some code duplication, but that's the only way I could do it, now that checker
  and non-checker options are separated. Note that the non-checker version
  inserts the new option to the ConfigTable with the default value, but the
  checker version only attempts to find already existing entries. This is how
  it always worked, but this is clunky and I might end reworking that too, so we
  can eventually get a ConfigTable that contains the entire configuration of the
  analyzer.

Differential Revision: https://reviews.llvm.org/D53483

llvm-svn: 346113