[Preprocessor] Implement -fminimize-whitespace.

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
changed (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, 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 ...