[dsymutil] Escape HTML special characters in plist.

When printing string in the Plist, we weren't escaping the characters
which lead to invalid XML. This patch adds the escape logic to
StringExtras.

rdar://39785334

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333565 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed