[llvm-cov] Optionally use a symbol demangler when preparing reports

[llvm-cov] Optionally use a symbol demangler when preparing reports

Add an option to specify a symbol demangler (as well as options to the
demangler). This can be used to make reports more human-readable.

This option is especially useful in -output-dir mode, since it isn't as
easy to manually pipe reports into a demangler in this mode.

llvm-svn: 275640