Bump the default clang version to 10.0.0

To match the current value in llvm-project. This fixes runnign clang
tests under build-script without explicitly setting the version, where
there is a test passing -fclang-abi-compat=9 that was previously
failing.

rdar://60376468

Author: benlangmuir

utils/build_swift/build_swift/defaults.py

@@ -48,7 +48,7 @@
 
 COMPILER_VENDOR = 'none'
 SWIFT_USER_VISIBLE_VERSION = Version('5.3')
-CLANG_USER_VISIBLE_VERSION = Version('7.0.0')
+CLANG_USER_VISIBLE_VERSION = Version('10.0.0')
 SWIFT_ANALYZE_CODE_COVERAGE = 'false'
 
 DARWIN_XCRUN_TOOLCHAIN = 'default'