cmStringCommand: clear intermediate matches
When `string(REGEX REPLACE)` or `string(REGEX MATCHALL)` loop internally, they store their matches, but they do not clear the previous match from an earlier iteration. This can leave the contents of `CMAKE_MATCH_<N>` with bogus values for later matches in the string if they have groups which earlier matched a non-empty string, but now match an empty string. Fixes #17079.
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