Add secondary libstdc++ 4.8 and 5.1 detection mechanisms

[hubert.reinterpretcast]

Summary

The date-based approach to detecting unsupported versions of libstdc++ does not handle bug fix releases of older versions. As an example, the __GLIBCXX__ value associated with version 5.1, 20150422, is less than the values associated with versions 4.8.5 and 4.9.3.

This patch adds secondary checks based on certain properties in sufficiently new versions of libstdc++.

[hubert.reinterpretcast]

A friendly ping?

[jfb]

I'm not a fan of this type of detection, but unfortunately it's the best we have for now. Can you add a comment to that effect, and mention that when we go to GCC 7.1 or later we'll be able to use a better solution because of this;

_GLIBCXX_RELEASE
The major release number for libstdc++. This macro is defined to the GCC major version that the libstdc++ headers belong to, as an integer constant. When compiling with GCC it has the same value as GCC's pre-defined macro __GNUC__. This macro can be used when libstdc++ is used with a non-GNU compiler where __GNUC__ is not defined, or has a different value that doesn't correspond to the libstdc++ version. This macro first appeared in the GCC 7.1 release and is not defined for GCC 6.x or older releases.

I can't validate your heuristics, but I assume they're correct.

[hubert.reinterpretcast]

Address review comment: Add note re: _GLIBCXX_RELEASE

cmake/modules/CheckCompilerVersion.cmake

if (CMAKE_CXX_SIMULATE_VERSION VERSION_LESS MSVC_MIN)
       message(FATAL_ERROR "Host Clang must have at least -fms-compatibility-version=${MSVC_MIN}, your version is ${CMAKE_CXX_COMPILER_VERSION}.")
     endif()
     set(CLANG_CL 1)
   elseif(NOT LLVM_ENABLE_LIBCXX)
     # Test that we aren't using too old of a version of libstdc++.
     set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
     set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++0x")
+    # Test for libstdc++ version of at least 4.8 by checking for _ZNKSt17bad_function_call4whatEv.
+    # Note: We should check _GLIBCXX_RELEASE when possible (i.e., for GCC 7.1 and up).
     check_cxx_source_compiles("
 #include <iosfwd>
 #if defined(__GLIBCXX__)
 #if __GLIBCXX__ < ${GCC_MIN_DATE}
 #error Unsupported libstdc++ version
 #endif
 #endif
-int main() { return 0; }
+#if defined(__GLIBCXX__)
+extern const char _ZNKSt17bad_function_call4whatEv[];
+const char *chk = _ZNKSt17bad_function_call4whatEv;
+#else
+const char *chk = \"\";
+#endif
+int main() { ++chk; return 0; }
 "
       LLVM_LIBSTDCXX_MIN)
     if(NOT LLVM_LIBSTDCXX_MIN)
       message(FATAL_ERROR "libstdc++ version must be at least ${GCC_MIN}.")
     endif()
+    # Test for libstdc++ version of at least 5.1 by checking for std::iostream_category().
+    # Note: We should check _GLIBCXX_RELEASE when possible (i.e., for GCC 7.1 and up).
     check_cxx_source_compiles("
 #include <iosfwd>
 #if defined(__GLIBCXX__)
 #if __GLIBCXX__ < ${GCC_SOFT_ERROR_DATE}
 #error Unsupported libstdc++ version
 #endif
 #endif
+#if defined(__GLIBCXX__)
+#include <ios>
+void foo(void) { (void) std::iostream_category(); }
+#endif
 int main() { return 0; }
 "
       LLVM_LIBSTDCXX_SOFT_ERROR)
     if(NOT LLVM_LIBSTDCXX_SOFT_ERROR)
       if(LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN)
         message(WARNING "libstdc++ version should be at least ${GCC_SOFT_ERROR} because LLVM will soon use new C++ features which your toolchain version doesn't support. Ignoring because you've set LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN, but very soon your toolchain won't be supported.")
       else()
         message(FATAL_ERROR "libstdc++ version should be at least ${GCC_SOFT_ERROR} because LLVM will soon use new C++ features which your toolchain version doesn't support. You can temporarily opt out using LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN, but very soon your toolchain won't be supported.")
       endif()
     endif()
     set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
     set(CMAKE_REQUIRED_LIBRARIES ${OLD_CMAKE_REQUIRED_LIBRARIES})
   endif()
 endif()