Explicitly drop support for old clang versions on Windows (GH-15415)

* Explicitly drop support for old clang versions on Windows

This ensures compatibility with the typedef redefinitions we use, and
makes it clear during the `configure` stage that old compiler versions
are not supported.

See <https://releases.llvm.org/> regarding the release history of clang.

We also remove support for two number versions of clang, since as of
LLVM 4.0.0 the project uses a three number versioning scheme; see
<https://blog.llvm.org/2016/12/llvms-new-versioning-scheme.html>.

Author: cmb69

win32/build/confutils.js

@@ -3073,8 +3073,9 @@ function toolset_get_compiler_version()
 		if (full.match(/clang version ([\d\.]+)/)) {
 			version = RegExp.$1;
 			version = version.replace(/\./g, '');
-			version = version/100 < 1 ? version*10 : version;
-
+			if (version < 400) {
+				ERROR("Building with clang " + version + " is no longer supported");
+			}
 			return version;
 		}
 	} else if (ICC_TOOLSET) {