Specify DWARF alignment in bits, not bytes.

In DWARF, alignment of types is specified in bits, as is made clear by the
parameter name `AlignInBits`. However, `rustc` was incorrectly passing a byte
alignment. This commit fixes that.

This was noticed in upstream LLVM when I tried to check in a test consisting of
LLVM IR generated from `rustc` and it triggered assertions [1].

[1]: https://reviews.llvm.org/D126835

src/test/codegen/debug-alignment.rs

@@ -0,0 +1,8 @@
+// Verifies that DWARF alignment is specified properly.
+//
+// compile-flags: -C debuginfo=2
+#![crate_type = "lib"]
+
+// CHECK: !DIGlobalVariable
+// CHECK: align: 32
+pub static A: u32 = 1;