[clang] Apply -fcoverage-prefix-map reverse order

This patch changes handling multiple -fcoverage-prefix-map options to
match GCC's behavior. GCC applies prefix remappings that are provided
in reverse order (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109591).

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D148757