[ADT][CMake][AutoConf] Fail-fast iterators for DenseMap

Summary:
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.

Enabling fail-fast iterators breaks the LLVM C++ ABI, so they are
predicated on `LLVM_ENABLE_ABI_BREAKING_CHECKS`.
`LLVM_ENABLE_ABI_BREAKING_CHECKS` by default flips with
`LLVM_ENABLE_ASSERTS`, but can be clamped to ON or OFF using the CMake /
autoconf build system.

Reviewers: chandlerc, dexonsmith, rnk, zturner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8351

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233310 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Config/config.h.in

@@ -417,6 +417,9 @@
 /* Installation directory for documentation */
 #undef LLVM_DOCSDIR
 
+/* Define to enable checks that alter the LLVM C++ ABI */
+#undef LLVM_ENABLE_ABI_BREAKING_CHECKS
+
 /* Define if threads enabled */
 #undef LLVM_ENABLE_THREADS
 

include/llvm/Config/llvm-config.h.cmake

@@ -101,4 +101,8 @@
 /* Define if we link Polly to the tools */
 #cmakedefine LINK_POLLY_INTO_TOOLS
 
+/* Define if LLVM is built with asserts and checks that change the layout of
+   client-visible data structures.  */
+#cmakedefine LLVM_ENABLE_ABI_BREAKING_CHECKS
+
 #endif

include/llvm/Config/llvm-config.h.in

@@ -98,4 +98,7 @@
 /* LLVM version string */
 #undef LLVM_VERSION_STRING
 
+/* Define to enable checks that alter the LLVM C++ ABI */
+#undef LLVM_ENABLE_ABI_BREAKING_CHECKS
+
 #endif