Fix `URLTemplate` stack overflow #997

Job	Run time
osx (macos-14)	2m 28s
linux-5_4 (swift:5.4-focal)	2m 59s
linux-5_5-plus (swift:5.5-focal)	3m 13s
osx (macos-15)	1m 55s
linux (swift:5.2-focal)	7m 46s
linux-5_4 (swift:5.4-centos8)	2m 52s
linux-5_5-plus (swift:5.5-centos8)	3m 6s
linux (swift:5.2-centos8)	6m 19s
linux-5_5-plus (swift:5.6-focal)	2m 42s
linux (swift:5.3-focal)	3m 34s
linux (swift:5.3-centos8)	3m 39s
linux-5_5-plus (swift:5.7-focal)	2m 40s
linux-5_5-plus (swift:5.7-jammy)	2m 45s
linux-5_5-plus (swift:5.8-focal)	2m 38s
linux-5_5-plus (swift:5.8-jammy)	2m 43s
linux-5_5-plus (swift:5.9-focal)	2m 48s
linux-5_5-plus (swift:5.9-jammy)	2m 58s
linux-5_5-plus (swift:5.10-focal)	2m 49s
linux-5_5-plus (swift:5.10-jammy)	2m 49s
linux-5_5-plus (swift:6.0-focal)	1m 36s
linux-5_5-plus (swift:6.0-jammy)	1m 33s
linux-5_5-plus (swift:6.0-noble)	1m 38s
linux-5_5-plus (swiftlang/swift:nightly-focal)	1m 58s
linux-5_5-plus (swiftlang/swift:nightly-jammy)	1m 56s
	1h 11m 24s

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