clang-format: Escape '*' in generated flag documentation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223118 91177308-0d34-0410-b5e6-96231b3b80d8
Reverted r223114, it caused failure on on clang-native-arm-cortex-a9.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223120 91177308-0d34-0410-b5e6-96231b3b80d8