[clang-doc] Fix html entities in rendered text

Replace &, <, >, ", and ' with their corresponding html entities in text rendered
by HTML generator.

Differential Revision: https://reviews.llvm.org/D65107

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@367045 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed