The Guard's Wait-Loop Rule Misses a Loop Keyword Glued to $' or a Backtick #2138

Problem

The guard's wait-loop rule never finds a loop keyword that its tokenizer glues to the syntax before it. Two shapes reach that on develop, and bash runs the loop in each of them.

• ANSI-C and locale quoting. The outer lex reads $'...' and $"..." as a $ glued to the quoted text, so the payload begins with $while, which is not a loop keyword. _marked_lex notices ANSI-C quoting, but that changes only which quote mask is used, not the tokens.
• A loop inside a backtick substitution. The tokenizer yields `while, which does not match while. The $(...) form of the same loop is denied.

Reproduce (constructed)

Each of these is allowed:

bash -c $'while true; do sleep 1; done'
eval $'while true; do sleep 1; done'
echo `while true; do sleep 1; done`

x=$(while true; do sleep 1; done) is denied.

Done looks like

A $'...' or $"..." word reads as the text bash gives it, and a backtick substitution's body is scanned as commands the way a $(...) body is. All three commands above are denied, and self-test rows pin them.

Found by the local strict review of the #2080 fix (handoff #2137). Each shape predates that fix, and the eval form reaches it once the fix reads eval payloads.