[build-script] Update the default CLANG_USER_VISIBLE_VERSION to 6.0.0

The swift-4.2-branch of Clang/LLVM is derived from LLVM 6.0. Set the
default Clang version to match.

rdar://problem/39960908

Author: bob-wilson

utils/build_swift/defaults.py

@@ -41,7 +41,7 @@
 
 COMPILER_VENDOR = 'none'
 SWIFT_USER_VISIBLE_VERSION = CompilerVersion('4.2')
-CLANG_USER_VISIBLE_VERSION = CompilerVersion('5.0.0')
+CLANG_USER_VISIBLE_VERSION = CompilerVersion('6.0.0')
 SWIFT_ANALYZE_CODE_COVERAGE = 'false'
 
 DARWIN_XCRUN_TOOLCHAIN = 'default'