pgsql: Avoid character classification in regex escape parsing.

Avoid character classification in regex escape parsing.

For regex escape sequences, just test directly for the relevant ASCII
characters rather than using locale-sensitive character
classification.

This fixes an assertion failure when a locale considers a non-ASCII
character, such as "൧", to be a digit.

Reported-by: Richard Guo
Discussion: https://postgr.es/m/CAMbWs49Q6UoKGeT8pBkMtJGJd+16CBFZaaWUk9Du+2ERE5g_YA@mail.gmail.com
Backpatch-through: 11

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c04c6c5d6f5f213f217f1e356bf1c2d8b4e255a2

Modified Files
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src/backend/regex/regc_lex.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)