[YAML] Plain scalars can not begin with most indicators.

Summary:
Discovered when clangd loads YAML symbols, some symbol documentations
start with indicators (e.g. "-"), but YAML prints them as plain scalars
(no quotes), which make the YAML parser fail to parse.

For these kind of strings, we need quotes.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, ioeric, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D42362

llvm-svn: 323097