Issue Description:

When parsing constructs like #sourceLocation(file:) or @available(message:), the parser uses the function Parser::getStringLiteralIfNotInterpolated() to extract the text from the literal. This function doesn't call Lexer::getEncodedStringSegment() to process backslash escapes in the string—it simply extracts the source file's text directly. However, the lexer still assumes that the string literal will have its escapes processed, so it rejects "invalid" escapes.

This has a number of bad effects. The worst is that there's no way to specify a #sourceLocation with a backslashed Windows path:

#sourceLocation(file: "a\b.swift", line: 10)    // error: invalid escape sequence in literal

#sourceLocation(file: "a\\b.swift", line: 10)
#error("where am I?")    // a\\b.swift:10:8: error: where am I?

But it also means that we don't process escapes in @_private(sourceFile:) import, @available(..., message:), #assert, etc.

We should correct these bugs by updating Parser::getStringLiteralIfNotInterpolated() to extract the string's contents with Lexer::getEncodedStringSegment(). However, this will require us to change how Parser::getStringLiteralIfNotInterpolated() and perhaps its callers manage memory, because they currently assume that the StringRef is backed directly by a source buffer and will never be freed.