Add char8_t support (C++20)

This patch adds support for the char8_t type introduced in C++20
char8_t. The original patch was submitted by James Blachly  on the LLDB
mailing list [1]. I modified the patch a bit and added a test.

[1] http://lists.llvm.org/pipermail/lldb-dev/2019-August/015393.html

Differential revision: https://reviews.llvm.org/D66447

llvm-svn: 369582