Merge pull request #16378 from bob-wilson/rdar39960908-42

[build-script] Update the default CLANG_USER_VISIBLE_VERSION to 6.0.0

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'