Fixes ziglang#12685.

The problem was that paths in the cache are assumed to endwithout the path separator, but are added verbatim from the command line
in several places. The solution here is to make sure that all prefixes are without path separator by cutting it off when it is passed
to Cache.addPrefix. This way, the assumption is met.

src/Cache.zig

@@ -31,10 +31,21 @@ const Compilation = @import("Compilation.zig");
 const log = std.log.scoped(.cache);
 
 pub fn addPrefix(cache: *Cache, directory: Compilation.Directory) void {
-    if (directory.path) |p| {
+    var patched_dir = directory;
+    if (patched_dir.path) |*path| {
+        if (std.mem.endsWith(u8, path.*, std.fs.path.sep_str)) {
+            // remove the path separator at the end. This might remove the "/" from an
+            // absolute path, and push "" into the cache, but this is acceptible,
+            // as both "/" and "" are semantically the same prefix of a unix path.
+            path.* = path.*[0 .. path.*.len - 1];
+        }
+    }
+
+    if (patched_dir.path) |p| {
         log.debug("Cache.addPrefix {d} {s}", .{ cache.prefixes_len, p });
     }
-    cache.prefixes_buffer[cache.prefixes_len] = directory;
+
+    cache.prefixes_buffer[cache.prefixes_len] = patched_dir;
     cache.prefixes_len += 1;
 }
 
@@ -73,7 +84,6 @@ fn findPrefixResolved(cache: *const Cache, resolved_path: []u8) !PrefixedPath {
     while (i < prefixes_slice.len) : (i += 1) {
         const p = prefixes_slice[i].path.?;
         if (mem.startsWith(u8, resolved_path, p)) {
-            // +1 to skip over the path separator here
             const sub_path = try gpa.dupe(u8, resolved_path[p.len + 1 ..]);
             gpa.free(resolved_path);
             return PrefixedPath{