Fix true exhaustivity warnings in Bootstrap

[liufengyun]

The false positives have been fixed in #2424, the following list are the true positives that need changes in the code:

• scala/tools/nsc/backend/jvm/BTypes.scala:337 (Fix exhaustivity check of classOrArrayType lampepfl/scala#19)
• compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala:362
• compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala:354
• doc-tool/src/dotty/tools/dottydoc/model/comment/HtmlParsers.scala:123
• doc-tool/src/dotty/tools/dottydoc/staticsite/tags.scala:71
• doc-tool/src/dotty/tools/dottydoc/util/MemberLookup.scala:71