stage2: detect redundant C/C++ source files

[andrewrk]

Cache exposes BinDigest.

Compilation gains a set of a BinDigest for every C/C++ source file. We
detect when the same source/flags have already been added and emit a
compile error. This prevents a deadlock in the caching system.

Closes #7308

Example output:

[nix-shell:~/dev/zig/build]$ ./zig build-lib foo.c foo.c
foo.c:1:1: error: unable to build C object: the same source file was already added to the same compilation with the same flags

The :1:1 thing is a separate issue, which can be addressed separately now that 2ed1ed9 introduced different kinds of error messages.