[PATCH] [ADT][CMake][AutoConf] fail-fast iterators for DenseMap

[Sanjoy Das]

Hi chandlerc, dexonsmith, rnk, zturner,

This is a re-do of D7931 with the following changes:

 * the `EpochTracker` class is now conditional on
   `LLVM_ENABLE_FAIL_FAST_ITERATORS` and not `NDEBUG`.

 * **The most significant change**: `LLVM_ENABLE_FAIL_FAST_ITERATORS`
   can be forced to be set or unset independent of `NDEBUG`.  By
   default, cmake and autoconf set it (thus enabling fail-fast
   iterators) for with-asserts builds and unset it for without-asserts
   builds.

With these changes, projects that require ABI compatibility between
`defined(NDEBUG)` builds and `!defined(NDEBUG)` builds can get that by
clamping `LLVM_ENABLE_FAIL_FAST_ITERATORS` to on or off.

== Minor changes from D7931 ==

 * `HandleBase::getEpochAddress` returns a `void *` to emphasise that
   the result is for comparison, not derefernecing.

 * `HandleBase::getEpochAddress` was missing in the fail-fast-disabled
   version of `HandleBase`.  This was not a problem earlier since the
   function is only called from `assert`s and when `assert`s were
   enabled, we'd always choose the fail-fast-enabled version of
   `HandleBase`.  Since this is no longer the case (a with-asserts
   build may be using the fail-fast-disabled version of `HandleBase`),
   the missing method shows up as an obvious compiler error.

Original summary for D7931:
This patch is an attempt at making `DenseMapIterator`s "fail-fast".
Fail-fast iterators that have been invalidated due to insertion into
the host `DenseMap` deterministically trip an assert (in debug mode)
on access, instead of non-deterministically hitting memory corruption
issues.

http://reviews.llvm.org/D8351

Files:
  CMakeLists.txt
  Makefile.config.in
  Makefile.rules
  autoconf/configure.ac
  cmake/modules/HandleLLVMOptions.cmake
  configure
  docs/CMake.rst
  include/llvm/ADT/DenseMap.h
  include/llvm/ADT/EpochTracker.h

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/
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