Don't warn about ASCII encodings (subset of UTF-8)

Currently warnings are thrown by concatenate-source-op every time ASDF
is loaded if UTF-8 is the default encoding because the ASDF source is
ASCII which is not equal to UTF-8.  This commit adds a special case to
the encoding equality check so that ASCII is recognized as a subset of
UTF-8.
7 jobs for !157 with better-component-encoding-warnings in 34 minutes and 45 seconds (queued for 1 second)
detached
Status Name Job ID Coverage
  Build
passed Build ASDF #9188

00:02:42

passed Build docs #9189

00:04:57

 
  Regression
passed ABCL 1.7.1 regression tests #9191

00:32:00

failed ABCL regression tests #9190
allowed to fail

00:29:22

passed CCL regression tests #9192

00:06:11

passed ECL regression tests #9193

00:06:55

passed SBCL regression tests #9194

00:05:02

 
Name Stage Failure
failed
ABCL regression tests Regression
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