fix: Fix process_preprocessed_file bug for distcc marker

fix: Fix process_preprocessed_file bug for distcc marker

2044fea84b86001b2976ecde946d3d6d0e88ec0a (included in ccache 4.6)
removed the special-casing of distcc’s pump in
process_preprocessed_file. That in turn revealed a bug that has been
present since 432d1ca6a6aa51f708124172169073c399fb68d2 (included in
ccache 3.4) but previously only affected compilations with distcc-pump
as the “compiler”: the detection and handling of distcc-pump’s
“__________” messages is broken in two ways:

1. It throws away everything between the last preprocessor directive and
   the “__________” marker. Thus, changes to such sections will not be
   included in the hash.
2. It detects “__________” markers in the middle of lines, not only at
   the beginning of lines.