[SR-10011] [Lexer] Raw Strings escape character sequence resembling multiline delimiter

[maustinstar]

Summary

The Lexer currently treats triple-quotes as a multiline delimiter for Strings. This PR allows the multiline delimiter (""") to be escaped in raw Strings when a valid closing custom delimiter (e.g. #) is found before a newline.

Problem

The Lexer does not register some strings that are technically valid according to the documented regex on string literals.

For example: #"""#
Gives the errors: Multi-line string literal content must begin on a new line & Unterminated string literal

Impact

This change does not impact any Strings that are already valid, rather, this PR enables a new set of String literals to be considered valid.

After merging this PR:
#"""# is now equivalent to "\""
#"""text"# equates to "\"\"text"
#""" is still the beginning of a multiline raw string.

Links

Resolves SR-10011.
Replaces my previous PR attempt: #23115

Tagging @rintaro and @xwu from activity in my previous PR #23115.

[rintaro]

@swift-ci Please smoke test

[maustinstar]

Error from the CI:

/Users/buildnode/jenkins/workspace/swift-PR-osx-smoke-test/branch-main/swift/test/Interop/Cxx/templates/class-template-variadic.swift:17:18: error: missing arguments for parameters #1, #2 in call
14:46:22   var pair = Pair()
14:46:22                  ^
14:46:22                   <#IntWrapper#>, <#IntWrapper#>
14:46:22 ClassTemplateVariadic.__CxxTemplateInst5TupleIJ10IntWrapperS0_EE:3:12: note: 'init(_:_:)' declared here
14:46:22     public init(_ t: IntWrapper, _ ts: IntWrapper)
14:46:22            ^
14:46:22 
14:46:22 --

This doesn't seem to be related to my PR. On my machine, the compiler builds and passes all tests using ninja. Did a recent merge break the compiler? Or, is this from my PR?

[xwu]

A recent merge; should be reverted now. Let's try again:

@swift-ci Please smoke test

[rintaro]

@swift-ci Please test Windows

[rintaro]

@swift-ci Please test Windows

[rintaro]

Thank you!