Handle line splicing (backslash-newline) in TokenList::Stream #741

simplecpp removes a backslash-newline only when it is between tokens. A splice inside a token splits the token instead of joining it.
The standard does line splicing in translation phase 2, before tokenization, so the token must be joined.

Proposed change

Remove backslash-newline in TokenList::Stream::readChar(), using the lookahead buffer from #739, and count the removed lines.
readfile adds the count to the line number at the next newline, as it does now for splices between tokens. The splice code in
readfile, readLineComment() and readErrorText() can then be removed or simplified.

Must stay unchanged

• Raw token locations. Tokens after a splice keep the line of the start of the logical line (cppcheck uses these locations, for
  example for inline suppressions).
• Splices between tokens, in string and character literals, in comments, and in #define bodies.
• Raw string literals: the splice is kept, so R"(a\⏎b)" still contains the backslash and the newline (C++ [lex.pptoken]).
• \ followed by spaces or tabs and a newline is still a splice, and still gives the "Combination 'backslash space newline' is not
  portable." warning.

Testing

• Tests for the cases in the table above, with all input types (including FileStream through readfileFromFile()).
• Compare -l output and raw tokens before and after for real code (/usr/include, cppcheck sources, simplecpp testsuite) and CRLF,
  CR, BOM and UTF-16 copies of them, with all 4 -input types.

Part of #708.