Bump Swift version to 6.5

[lorentey]

Now that we have opened release/6.4.x, we need to be able to easily distinguish between toolchain builds produced from main vs the new release branch, and we need to be able to introduce API and language/runtime features that target main only.

Bump the nominal Swift version to 6.5, and define an availability macro for SwiftStdlib 6.5. Note that this is a provisional version number, serving as a technical placeholder that the project may decide to change. If that happens, then we can simply update these definitions as needed, similar to how Swift 5.11 got changed to Swift 6.0 during its development.

[lorentey]

@swift-ci test

[lorentey]

Related package manager change: swiftlang/swift-package-manager#10032

swift-format appears to have automated this process (swiftlang/swift-format#1199)

I expect we'll also need a similar version update in swift-testing (cc @grynspan), and possibly elsewhere (?)

[grynspan]

Swift Testing has an issue in place to automate the process.

[lorentey]

@swift-ci smoke test

[lorentey]

11:36:04 
 ******************** TEST 'Swift-validation(linux-x86_64) :: SILOptimizer/many_struct_properties_scale_test.swift.gyb' FAILED ********************
11:36:04  Exit Code: 0
11:36:04  
11:36:04  Command Output (stdout):
11:36:04  --
11:36:04  # RUN: at line 1
11:36:04  /usr/bin/python3.10 /home/build-user/swift/utils/scale-test --swiftc-binary=/home/build-user/build/buildbot_linux/swift-linux-x86_64/bin/swiftc --tmpdir=/home/build-user/build/buildbot_linux/swift-linux-x86_64/validation-test-linux-x86_64/SILOptimizer/Output/many_struct_properties_scale_test.swift.gyb.tmp --begin 50 --end 1000 --superlinear-threshold 5 --trace --select SIL-processing.user -O /home/build-user/swift/validation-test/SILOptimizer/many_struct_properties_scale_test.swift.gyb
11:36:04  # executed command: /usr/bin/python3.10 /home/build-user/swift/utils/scale-test --swiftc-binary=/home/build-user/build/buildbot_linux/swift-linux-x86_64/bin/swiftc --tmpdir=/home/build-user/build/buildbot_linux/swift-linux-x86_64/validation-test-linux-x86_64/SILOptimizer/Output/many_struct_properties_scale_test.swift.gyb.tmp --begin 50 --end 1000 --superlinear-threshold 5 --trace --select SIL-processing.user -O /home/build-user/swift/validation-test/SILOptimizer/many_struct_properties_scale_test.swift.gyb
11:36:04  # .---command stdout------------
11:36:04  # | running: /home/build-user/build/buildbot_linux/swift-linux-x86_64/bin/swiftc -frontend -c -o out.o -O -primary-file in0.swift
11:36:04  # | running: /home/build-user/build/buildbot_linux/swift-linux-x86_64/bin/swiftc -frontend -c -o out.o -O -primary-file in50.swift
11:36:04  # | running: /home/build-user/build/buildbot_linux/swift-linux-x86_64/bin/swiftc -frontend -c -o out.o -O -primary-file in1000.swift
11:36:04  # | values: [6822.0, 71013.0, 3889718.0]
11:36:04  # | factor: 3.0244862987023104
11:36:04  # `-----------------------------
11:36:04  # .---command stderr------------
11:36:04  # | swift runtime: unknown backtracing setting 'warnings'
11:36:04  # | swift runtime: unknown backtracing setting 'warnings'
11:36:04  # | swift runtime: unknown backtracing setting 'warnings'
11:36:04  # | swift runtime: unknown backtracing setting 'warnings'
11:36:04  # `-----------------------------
11:36:04  
11:36:04  --
11:36:04  
11:36:04  ********************

[lorentey]

@eeckstein SILOptimizer/many_struct_properties_scale_test.swift.gyb seems to be failing in smoke tests

[xavgru12]

@swift-ci smoke test

[xavgru12]

@swift-ci smoke test macos

[lorentey]

@swift-ci smoke test