[6.1 cherry-pick] Allow string-interpolating fatalError, assert, assertionFailure, precondition, preconditionFailure in Embedded Swift

[kubamracek]

Cherry-pick of #79581 and #79698 and #79546 to 6.1.

• Explanation: Embedded Swift has in the past only supported static strings in fatalError() and similar calls. This adds back the ability to use strings and string interpolations, which is a high-value addition (allows logging of values that caused failures).
• Scope: Embedded Swift users only.
• Issues: N/A
• Original PRs: [embedded] Allow string-interpolating fatalError in Embedded Swift #79581 and [embedded] Allow string-interpolatings in assert, assertionFailure, precondition, preconditionFailure #79698 and [IRGen] Expose an -Xfrontend flag to disable the LLVM MergeFunctions pass #79546
• Reviewed By: @phausler + @aschwaighofer
• Risk: Low; changes are only in the Embedded build of the stdlib
• Testing: N/A

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test