[clang-format] Add a test showing the current config file list parsing

[arichardson]

Summary

Currently clang-format starts overriding the default values at index 0
(keeping the existing values) instead of appending or replacing all values.
This patch simply checks the current (IMO surprising) behaviour and does
not attempt to change it.

[MyDeveloperDay]

LGTM

clang/test/Format/dump-config-list-override.cpp

+/// Check that the ForEachMacros, etc. config entries replace default values instead of appending
+/// FIXME: clang-format currently start overriding at index 0 (keeping the remaining
+/// values) instead of either appending or completely replacing the values.
+/// This behaviour is highly confusing. For now this test documents the current state.
+// RUN: clang-format -style="{BasedOnStyle: LLVM}" -dump-config %s | \
+// RUN:   FileCheck %s --check-prefixes=CHECK,DEFAULT
+// RUN: clang-format -style="{BasedOnStyle: LLVM, ForEachMacros: ['OVERRIDE_FOREACH']}" -dump-config %s | \
+// RUN:   FileCheck %s --check-prefixes=CHECK,OVERRIDE,FIXME-SHOULD-NOT-BE
+// RUN: clang-format -style="{BasedOnStyle: LLVM, ForEachMacros: ['M1', 'M2', 'M3', 'M4']}" -dump-config %s | \
+// RUN:   FileCheck %s --check-prefixes=CHECK,MORE-ENTRIES-THAN-DEFAULT
+
+
+// CHECK-LABEL:   ForEachMacros:
+// DEFAULT-NEXT:  {{^  }}- foreach
+// DEFAULT-NEXT:  {{^  }}- Q_FOREACH
+// DEFAULT-NEXT:  {{^  }}- BOOST_FOREACH
+// OVERRIDE-NEXT: {{^  }}- OVERRIDE_FOREACH
+// FIXME-SHOULD-NOT-BE-NEXT:  {{^  }}- Q_FOREACH
+// FIXME-SHOULD-NOT-BE-NEXT:  {{^  }}- BOOST_FOREACH
+// MORE-ENTRIES-THAN-DEFAULT-NEXT: {{^  }}- M1
+// MORE-ENTRIES-THAN-DEFAULT-NEXT: {{^  }}- M2
+// MORE-ENTRIES-THAN-DEFAULT-NEXT: {{^  }}- M3
+// MORE-ENTRIES-THAN-DEFAULT-NEXT: {{^  }}- M4
+// CHECK-NEXT:    {{^[F-Z]}}