Preserve, clarify, and extend debug information

`-Cdebuginfo=1` was never line tables only and
can't be due to backwards compatibility issues.
This was clarified and an option for line tables only
was added. Additionally an option for line info
directives only was added, which is well needed for
some targets. The debug info options should now
behave the same as clang's debug info options.

tests/codegen/debug-line-tables-only.rs

@@ -0,0 +1,28 @@
+// Verify that the only debuginfo generated are the line tables.
+//
+// ignore-windows
+// compile-flags: -C debuginfo=line-tables-only
+
+#[repr(C)]
+struct StructType {
+    a: i64,
+    b: i32
+}
+
+extern "C" {
+    fn creator() -> *mut StructType;
+    fn save(p: *const StructType);
+}
+
+fn main() {
+    unsafe {
+        let value: &mut StructType = &mut* creator();
+        value.a = 7;
+        save(value as *const StructType)
+    }
+}
+
+// CHECK: !DICompileUnit
+// CHECK: emissionKind: LineTablesOnly
+// CHECK: !DILocation
+// CHECK-NOT: !DIBasicType