Revert "Temporarily bump minimum compiler version"

It's been in for more than 30 min and no bots have complained. Let's see if some
slow ones catch up. I'll do another manual pass on bots later (in case some that
were down are back up), and then turn this on permanently through a regular
review.

llvm-svn: 368253
diff --git a/llvm/cmake/modules/CheckCompilerVersion.cmake b/llvm/cmake/modules/CheckCompilerVersion.cmake
index ad70d85..2f4d75f 100644
--- a/llvm/cmake/modules/CheckCompilerVersion.cmake
+++ b/llvm/cmake/modules/CheckCompilerVersion.cmake
@@ -4,11 +4,11 @@
 
 include(CheckCXXSourceCompiles)
 
-set(GCC_MIN 5.1)
+set(GCC_MIN 4.8)
 set(GCC_SOFT_ERROR 5.1)
-set(CLANG_MIN 3.5)
+set(CLANG_MIN 3.1)
 set(CLANG_SOFT_ERROR 3.5)
-set(APPLECLANG_MIN 6.0)
+set(APPLECLANG_MIN 3.1)
 set(APPLECLANG_SOFT_ERROR 6.0)
 
 # https://en.wikipedia.org/wiki/Microsoft_Visual_C#Internal_version_numbering
@@ -17,7 +17,7 @@
 set(MSVC_SOFT_ERROR 19.1)
 
 # Map the above GCC versions to dates: https://gcc.gnu.org/develop.html#timeline
-set(GCC_MIN_DATE 20150422)
+set(GCC_MIN_DATE 20130322)
 set(GCC_SOFT_ERROR_DATE 20150422)