Remove minimum toolchain soft-error

[jfb]

Summary

Back in January I changed the minimum toolchain version required to build clang and LLVM: D57264. Since then we've release LLVM 8, following our process it's therefore now a good time to remove the soft-error and officially deprecate older toolchains. I tried this out last Tursday night to see if any bots complained, and I saw no complaints. I also manually audited bots and didn't see any bot that should break, but their toolchain information is unreliable and some bots are offline.

Once this patch stick we'll move to C++14 as we've already agreed.

[chandlerc]

LGTM.

We should specifically call this out in release notes as well (before we forget) as a bunch of downstream people will discover it in LLVM 10.

llvm/trunk/cmake/modules/CheckCompilerVersion.cmake

 # Check if the host compiler is new enough.
 # These versions are updated based on the following policy:
 #   llvm.org/docs/DeveloperPolicy.html#toolchain
 
 include(CheckCXXSourceCompiles)
 
-set(GCC_MIN 4.8)
+set(GCC_MIN 5.1)
 set(GCC_SOFT_ERROR 5.1)
-set(CLANG_MIN 3.1)
+set(CLANG_MIN 3.5)
 set(CLANG_SOFT_ERROR 3.5)
-set(APPLECLANG_MIN 3.1)
+set(APPLECLANG_MIN 6.0)
 set(APPLECLANG_SOFT_ERROR 6.0)
 
 # https://en.wikipedia.org/wiki/Microsoft_Visual_C#Internal_version_numbering
 # _MSC_VER == 1910 MSVC++ 14.1 (Visual Studio 2017 version 15.0)
 set(MSVC_MIN 19.1)
 set(MSVC_SOFT_ERROR 19.1)
 
 # Map the above GCC versions to dates: https://gcc.gnu.org/develop.html#timeline
-set(GCC_MIN_DATE 20130322)
+set(GCC_MIN_DATE 20150422)
 set(GCC_SOFT_ERROR_DATE 20150422)
 
 
 if(DEFINED LLVM_COMPILER_CHECKED)
   return()
 endif()
 set(LLVM_COMPILER_CHECKED ON)
 

llvm/trunk/docs/GettingStarted.rst

 standards<CodingStandards>`. To enforce this language version, we check the most
 popular host toolchains for specific minimum versions in our build systems:
 
 * Clang 3.5
 * Apple Clang 6.0
 * GCC 5.1
 * Visual Studio 2017
 
-The below versions currently soft-error as we transition to the new compiler
-versions listed above. The LLVM codebase is currently known to compile correctly
-with the following compilers, though this will change in the near future:
-
-* Clang 3.1
-* Apple Clang 3.1
-* GCC 4.8
-* Visual Studio 2017
-
 Anything older than these toolchains *may* work, but will require forcing the
 build system with a special option and is not really a supported host platform.
 Also note that older versions of these compilers have often crashed or
 miscompiled LLVM.
 
 For less widely used host toolchains such as ICC or xlC, be aware that a very
 recent version may be required to support all of the C++ features used in LLVM.