[test] Fix plugin tests

Recommit of r361790 that was temporarily reverted in r361793 due to bot breakage.

Summary:
The following changes were required to fix these tests:

1) Change LLVM_ENABLE_PLUGINS to an option and move it to
   llvm/CMakeLists.txt with an appropriate default -- which matches
   the original default behavior.

2) Move the plugins directory from clang/test/Analysis
   clang/lib/Analysis.  It's not enough to add an exclude to the
   lit.local.cfg file because add_lit_testsuites recurses the tree and
   automatically adds the appropriate `check-` targets, which don't
   make sense for the plugins because they aren't tests and don't
   have `RUN` statements.

   Here's a list of the `clang-check-anlysis*` targets with this
   change:

```
  $ ninja -t targets all| sed -n "s/.*\/\(check[^:]*\):.*/\1/p" | sort -u | grep clang-analysis
  check-clang-analysis
  check-clang-analysis-checkers
  check-clang-analysis-copypaste
  check-clang-analysis-diagnostics
  check-clang-analysis-engine
  check-clang-analysis-exploration_order
  check-clang-analysis-html_diagnostics
  check-clang-analysis-html_diagnostics-relevant_lines
  check-clang-analysis-inlining
  check-clang-analysis-objc
  check-clang-analysis-unified-sources
  check-clang-analysis-z3
```

3) Simplify the logic and only include the subdirectories under
   clang/lib/Analysis/plugins if LLVM_ENABLE_PLUGINS is set.

Reviewed By: NoQ

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D62445

llvm-svn: 362328