Define placeholder availability for Swift 6.2

[lorentey]

This PR defines SwiftStdlib 6.2 to track availability of features introduced on the current main branch.

SwiftStdlib 6.2:macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, visionOS 9999

It also updates RuntimeVersions.def to define placeholder availability for the 6.2 runtime within the compiler.

Additionally, _SwiftStdlibVersion gains a new .v6_2_0 case, and _SwiftStdlibVersion.current is updated to report that.

[lorentey]

@swift-ci test

[lorentey]

******************** TEST 'Swift(macosx-x86_64) :: abi/macOS/x86_64/stdlib-asserts.swift' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
--- /Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/swift/test/abi/macOS/x86_64/../../Inputs/macOS/x86_64/stdlib/baseline-asserts	2025-01-10 02:32:42
+++ /Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/build/buildbot_incremental/swift-macosx-x86_64/test-macosx-x86_64/abi/macOS/x86_64/Output/stdlib-asserts.swift.tmp/symbols	2025-01-10 04:04:46
@@ -3385,6 +3385,7 @@
 _$sSo19_SwiftStdlibVersionas23CustomStringConvertiblesWP
 _$sSo19_SwiftStdlibVersionasE11descriptionSSvg
 _$sSo19_SwiftStdlibVersionasE11descriptionSSvpMV
+_$sSo19_SwiftStdlibVersionasE6v6_2_0ABvpZMV
 _$sSo19_SwiftStdlibVersionasE7currentABvgZ
 _$sSp10deallocate8capacityySi_tF
 _$sSp10deallocateyyF

--

D'oh, I didn't realize we're exporting these for @_aeic definitions. 🤔

[lorentey]

@swift-ci test