DI: Reverse direction of subprogram -> function edge.

Previously, subprograms contained a metadata reference to the function they
described. Because most clients need to get or set a subprogram for a given
function rather than the other way around, this created unneeded inefficiency.

For example, many passes needed to call the function llvm::makeSubprogramMap()
to build a mapping from functions to subprograms, and the IR linker needed to
fix up function references in a way that caused quadratic complexity in the IR
linking phase of LTO.

This change reverses the direction of the edge by storing the subprogram as
function-level metadata and removing DISubprogram's function field.

Since this is an IR change, a bitcode upgrade has been provided.

Fixes PR23367. An upgrade script for textual IR for out-of-tree clients is
attached to the PR.

Differential Revision: http://reviews.llvm.org/D14265

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252219 91177308-0d34-0410-b5e6-96231b3b80d8

test/Bitcode/upgrade-subprogram.ll

@@ -0,0 +1,17 @@
+; RUN: llvm-dis < %s.bc | FileCheck %s
+; RUN: verify-uselistorder < %s.bc
+
+; CHECK: define void @foo() !dbg [[SP:![0-9]+]]
+define void @foo() {
+  ret void
+}
+
+!llvm.module.flags = !{!0}
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+
+!llvm.dbg.cu = !{!1}
+!1 = distinct !DICompileUnit(language: DW_LANG_C99, file: !2, subprograms: !{!3}, emissionKind: 1)
+!2 = !DIFile(filename: "foo.c", directory: "/path/to/dir")
+; CHECK: [[SP]] = distinct !DISubprogram
+!3 = distinct !DISubprogram(file: !2, scope: !2, line: 51, name: "foo", function: void ()* @foo, type: !4)
+!4 = !DISubroutineType(types: !{})