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/X86/misched-code-difference-with-debug.ll

@@ -86,5 +86,5 @@ declare void @llvm.dbg.value(metadata, i64, metadata, metadata)
 !21 = !{!"0x34\00argc\00argc\00\001\000\001", null, !14, !11, i8* @argc, null} ; [ DW_TAG_variable ] [argc] [line 1] [def]
 !22 = !{i32 2, !"Dwarf Version", i32 4}
 !23 = !{i32 2, !"Debug Info Version", i32 2}
-!25 = !{i32 8, i32 3, !13, null}
+!25 = !MDLocation(line: 8, column: 3, scope: !13)
 !29 = !{!"0x102"}               ; [ DW_TAG_expression ]