.debug_line incremental compilation initial support

Supports writing the first function. Still TODO is:
 * handling the .debug_line header growing too large
 * adding a new file to an existing compilation
 * adding an additional function to an existing file
 * handling incremental updates
 * adding the main IR debug ops for IR instructions

There are also issues to work out:
 * readelf --debug-dump=rawline is saying there is no .debug_str section
   even though there is
 * readelf --debug-dump=decodedline is saying the file index 0 is bad
   and reporting some other kind of corruption.

lib/std/hash_map.zig

@@ -196,6 +196,10 @@ pub fn HashMap(
             return self.unmanaged.getEntry(key);
         }
 
+        pub fn getIndex(self: Self, key: K) ?usize {
+            return self.unmanaged.getIndex(key);
+        }
+
         pub fn get(self: Self, key: K) ?V {
             return self.unmanaged.get(key);
         }
@@ -479,17 +483,21 @@ pub fn HashMapUnmanaged(
         }
 
         pub fn getEntry(self: Self, key: K) ?*Entry {
+            const index = self.getIndex(key) orelse return null;
+            return &self.entries.items[index];
+        }
+
+        pub fn getIndex(self: Self, key: K) ?usize {
             const header = self.index_header orelse {
                 // Linear scan.
                 const h = if (store_hash) hash(key) else {};
-                for (self.entries.items) |*item| {
+                for (self.entries.items) |*item, i| {
                     if (item.hash == h and eql(key, item.key)) {
-                        return item;
+                        return i;
                     }
                 }
                 return null;
             };
-
             switch (header.capacityIndexType()) {
                 .u8 => return self.getInternal(key, header, u8),
                 .u16 => return self.getInternal(key, header, u16),
@@ -711,7 +719,7 @@ pub fn HashMapUnmanaged(
             unreachable;
         }
 
-        fn getInternal(self: Self, key: K, header: *IndexHeader, comptime I: type) ?*Entry {
+        fn getInternal(self: Self, key: K, header: *IndexHeader, comptime I: type) ?usize {
             const indexes = header.indexes(I);
             const h = hash(key);
             const start_index = header.constrainIndex(h);
@@ -725,7 +733,7 @@ pub fn HashMapUnmanaged(
                 const entry = &self.entries.items[index.entry_index];
                 const hash_match = if (store_hash) h == entry.hash else true;
                 if (hash_match and eql(key, entry.key))
-                    return entry;
+                    return index.entry_index;
             }
             return null;
         }