Map name as known by PCH to name on disk. (ROOT-6865)

(cherry picked from commit e893dfcd3640e4ba4e20d6622c3a434f00003775)
When matching files to PCH, ignore leading "./" which can come from -I. (ROOT-6865)

(cherry picked from commit 6545d8c211ef5f8dda2d5739712d354dfdea68b3)
Force the PCH to provide the header even if date/size are off (ROOT-6865).

(cherry picked from commit c2df2101a9a83f41336d34c11c8f026eaab827e8)