This patch adds the -fminimize-whitespace with the following effects:
- If combined with -E, remove as much non-line-breaking whitespace as possible
- If combined with -E -P, removes as much whitespace as possible, including line-breaks.
The motivation is to reduce the amount of insignificant changes in the preprocessed output with source files where only whitespace has been changes (add/remove comments, clang-format, etc.) which is in particular useful with ccache.
A patch for ccache for using this flag has been proposed to ccache as well: https://github.com/ccache/ccache/pull/815, which will use -fnormalize-whitespace when clang-13 has been detected (assuming this will be merged before the release fork), and additionally uses -P in "unify_mode". ccache already had a unify_mode in an older version which was removed because of problems that using the preprocessor itself does not have (such that the custom tokenizer did not recognize C++11 raw strings).
This patch slightly reorganizes which part is responsible for adding newlines that are required for semantics. It is now either startNewLineIfNeeded() or MoveToLine() but never both; this avoids the ShouldUpdateCurrentLine workaround and avoids redundant lines being inserted in some cases. It also fixes a mandatory newline not inserted after a _Pragma("...") that is expanded into a #pragma.