[C++11] Clarify in the Compiler.h support macros and comments (and

assert it with an #error) that we require MSVC 2012; MSVC 2010 will no
longer suffice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202631 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Compiler.h

@@ -47,11 +47,16 @@
 /// \macro LLVM_MSC_PREREQ
 /// \brief Is the compiler MSVC of at least the specified version?
 /// The common \param version values to check for are:
-///  * 1600: Microsoft Visual Studio 2010 / 10.0
 ///  * 1700: Microsoft Visual Studio 2012 / 11.0
 ///  * 1800: Microsoft Visual Studio 2013 / 12.0
 #ifdef _MSC_VER
 #define LLVM_MSC_PREREQ(version) (_MSC_VER >= (version))
+
+// We require at least MSVC 2012.
+#if !LLVM_MSC_PREREQ(1700)
+#error LLVM requires at least MSVC 2012.
+#endif
+
 #else
 #define LLVM_MSC_PREREQ(version) 0
 #endif