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.
Add a string(CONCAT) command to simply concatenate input arguments
together. This will be useful for combining strings from different
quoting syntaxes. Add a RunCMake.string test covering these cases.
