Add a cmake option for using strcmp for type_infos.

[danalbert]

Summary

libc++ doesn't yet have the code for this, but libc++abi does. Adding
the switch to libc++ since the flag in libc++abi is
_LIBCXX_DYNAMIC_FALLBACK, not _LIBCXXABI_DYNAMIC_FALLBACK, and it
will be needed here as well.

[danalbert]

Actually, I was wrong. This is implemented. Will update to set the flag the configures this and add a test.

[danalbert]

zygoloid and nbjoerg got me pointed in the right direction on this. Looks like the user didn't have a key function defined for one of their classes, which was actually the root of the problem.

CMakeLists.txt

cmake_dependent_option(LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY
         "Install libc++experimental.a" ON
         "LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY;LIBCXX_INSTALL_LIBRARY" OFF)
 set(LIBCXX_ABI_VERSION 1 CACHE STRING "ABI version of libc++.")
 option(LIBCXX_ABI_UNSTABLE "Unstable ABI of libc++." OFF)
 option(LIBCXX_ABI_FORCE_ITANIUM "Ignore auto-detection and force use of the Itanium ABI.")
 option(LIBCXX_ABI_FORCE_MICROSOFT "Ignore auto-detection and force use of the Microsoft ABI.")
 set(LIBCXX_ABI_DEFINES "" CACHE STRING "A semicolon separated list of ABI macros to define in the site config header.")
 option(LIBCXX_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
+option(LIBCXX_DYNAMIC_FALLBACK  "Use strcmp fallback for comparing type_infos." OFF)
 
 if (NOT LIBCXX_ENABLE_SHARED AND NOT LIBCXX_ENABLE_STATIC)
   message(FATAL_ERROR "libc++ must be built as either a shared or static library.")
 endif()
 
 # ABI Library options ---------------------------------------------------------
 set(LIBCXX_CXX_ABI "default" CACHE STRING
     "Specify C++ ABI library to use.")