This patch improves the way lldb checks if the terminal it's opened in
(if any) supports Unicode or not.
On POSIX systems, we check if `LANG` contains `UTF-8`.
On Windows, we always return `true` since we use the `WriteToConsoleW`
api.
This is a relanding of https://github.com/llvm/llvm-project/pull/168603.
The tests failed because the bots support Unicode but the tests expect
ASCII. To avoid different outputs depending on the environment the tests
are running in, this patch always force ASCII in the tests.
This patch fixes issues introduced by
https://github.com/llvm/llvm-project/pull/171491 when running tests in
CI.
The shell tests expect certain characters when matching diagnostics.
With https://github.com/llvm/llvm-project/pull/171491, those characters
can either be Unicode specific characters or their ASCII equivalent. The
tests were always expecting the ASCII version. This patch fixes this by
using a regex to match one or the other.
