IR: Move MDLocation into place

This commit moves `MDLocation`, finishing off PR21433.  There's an
accompanying clang commit for frontend testcases.  I'll attach the
testcase upgrade script I used to PR21433 to help out-of-tree
frontends/backends.

This changes the schema for `DebugLoc` and `DILocation` from:

    !{i32 3, i32 7, !7, !8}

to:

    !MDLocation(line: 3, column: 7, scope: !7, inlinedAt: !8)

Note that empty fields (line/column: 0 and inlinedAt: null) don't get
printed by the assembly writer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226048 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/XCore/dwarf_debug.ll

@@ -35,5 +35,5 @@ declare void @llvm.dbg.declare(metadata, metadata, metadata)
 !9 = !{i32 2, !"Dwarf Version", i32 4}
 !10 = !{i32 2, !"Debug Info Version", i32 2}
 !11 = !{!"0x101\00a\0016777218\000", !4, !5, !8} ; [ DW_TAG_arg_variable ]
-!12 = !{i32 2, i32 0, !4, null}
+!12 = !MDLocation(line: 2, scope: !4)