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/DebugInfo/array.ll

@@ -31,8 +31,8 @@ declare void @llvm.dbg.declare(metadata, metadata, metadata) nounwind readnone
 ;CHECK-NOT: DW_AT_upper_bound
 ;CHECK-NEXT: End Of Children Mark
 !10 = !{!"0x21\000\00-1"}        ; [ DW_TAG_subrange_type ]
-!11 = !{i32 4, i32 7, !7, null}
-!12 = !{i32 5, i32 3, !7, null}
+!11 = !MDLocation(line: 4, column: 7, scope: !7)
+!12 = !MDLocation(line: 5, column: 3, scope: !7)
 !13 = !{!0}
 !14 = !{!"array.c", !"/private/tmp"}
 !15 = !{i32 0}