Pull google/benchmark library to the LLVM tree

This patch pulls google/benchmark v1.4.1 into the LLVM tree so that any
project could use it for benchmark generation. A dummy benchmark is
added to `llvm/benchmarks/DummyYAML.cpp` to validate the correctness of
the build process.

The current version does not utilize LLVM LNT and LLVM CMake
infrastructure, but that might be sufficient for most users. Two
introduced CMake variables:

* `LLVM_INCLUDE_BENCHMARKS` (`ON` by default) generates benchmark
  targets
* `LLVM_BUILD_BENCHMARKS` (`OFF` by default) adds generated
  benchmark targets to the list of default LLVM targets (i.e. if `ON`
  benchmarks will be built upon standard build invocation, e.g. `ninja` or
  `make` with no specific targets)

List of modifications:

* `BENCHMARK_ENABLE_TESTING` is disabled
* `BENCHMARK_ENABLE_EXCEPTIONS` is disabled
* `BENCHMARK_ENABLE_INSTALL` is disabled
* `BENCHMARK_ENABLE_GTEST_TESTS` is disabled
* `BENCHMARK_DOWNLOAD_DEPENDENCIES` is disabled

Original discussion can be found here:
http://lists.llvm.org/pipermail/llvm-dev/2018-August/125023.html

Reviewed by: dberris, lebedev.ri

Subscribers: ilya-biryukov, ioeric, EricWF, lebedev.ri, srhines,
dschuff, mgorny, krytarowski, fedor.sergeev, mgrang, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D50894

llvm-svn: 340809