[5.0][IDE][syntax-coloring] Treat unterminated raw strings as strings for syntax coloring purposes

[nathawes]

Cherry-pick of #22845

• Explanation: Unterminated string literals get lexed as unknown tokens. For syntax highlighting we check if an unknown token starts with a " and highlight it as a string in order to provide feedback to the user as soon as they start writing a string. This check doesn't work for raw string literals, though (e.g. #"I am "raw" "# or ##" I am #"raw"# "##) that were introduced in this release. This patch adjusts the check to trim off any leading # characters before checking for an opening quote when determining if an unknown token should be highlighted as a string.
• Scope: Affects syntax highlighting of raw strings the user has not yet terminated correctly
• Issue: rdar://problem/48269826
• Risk: Very low. This only affects the syntax highlighting of unknown tokens and the entire code change is modifying the existing check from Tok.getRawText().startswith("\"") to Tok.getRawText().ltrim('#').startswith("\"")
• Testing: Extended the syntax highlighting regression tests with unterminated raw strings, and valid raw strings with a varied number of leading/trailing # characters and interpolations, and all existing tests pass. Also manually verified raw string highlighting works as expected with these changes in Xcode.
• Reviewer: Xi Ge (in the master PR)

Resolves rdar://problem/48269826

[nathawes]

@swift-ci please test

[nathawes]

@swift-ci please nominate