feat: Support overriding MSVC /Z* options

feat: Support overriding MSVC /Z* options

MSVC options /Zi and /ZI are too hard since they produce separate PDB
files. /Z7 is OK, but if the command line contains /Zi or /ZI followed
by /Z7, MSVC will use the latter (with a warning) but ccache will still
consider the command line too hard.

This commit makes ccache understand that only the last /Z* option will
be used and thus accepts the command line if the last /Z* option is /Z7.

Closes #1239.