Revamp test-suite documentation

- Remove duplication: Both TestingGuide and TestSuiteMakefileGuide
  would give a similar overview over the test-suite.
- Present cmake/lit as the default/normal way of running the test-suite:
- Move information about the cmake/lit testsuite into the new
  TestSuiteGuide.rst file. Mark the remaining information in
  TestSuiteMakefilesGuide.rst as deprecated.
- General simplification and shorting of language.
- Remove paragraphs about tests known to fail as everything should pass
  nowadays.
- Remove paragraph about zlib requirement; it's not required anymore
  since we copied a zlib source snapshot into the test-suite.
- Remove paragraph about comparison with "native compiler". Correctness is
  always checked against reference outputs nowadays.
- Change cmake/lit quickstart section to recommend `pip` for installing
  lit and use `CMAKE_C_COMPILER` and a cache file in the example as that
  is what most people will end up doing anyway. Also a section about
  compare.py to quickstart.
- Document `Bitcode` and `MicroBenchmarks` directories.
- Add section with commonly used cmake configuration options.
- Add section about showing and comparing result files via compare.py.
- Add section about using external benchmark suites.
- Add section about using custom benchmark suites.
- Add section about profile guided optimization.
- Add section about cross-compilation and running on external devices.

Differential Revision: https://reviews.llvm.org/D51465

llvm-svn: 341260