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/Transforms/SafeStack/debug-loc.ll

@@ -24,7 +24,7 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; Function Attrs: safestack uwtable
-define void @f() #0 {
+define void @f() #0 !dbg !4 {
 entry:
   %buf = alloca [100 x i32], align 16
   %0 = bitcast [100 x i32]* %buf to i8*, !dbg !16
@@ -63,7 +63,7 @@ attributes #4 = { nounwind }
 !1 = !DIFile(filename: "1.cc", directory: "/tmp")
 !2 = !{}
 !3 = !{!4}
-!4 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 4, type: !5, isLocal: false, isDefinition: true, scopeLine: 4, flags: DIFlagPrototyped, isOptimized: true, function: void ()* @f, variables: !7)
+!4 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 4, type: !5, isLocal: false, isDefinition: true, scopeLine: 4, flags: DIFlagPrototyped, isOptimized: true, variables: !7)
 !5 = !DISubroutineType(types: !6)
 !6 = !{null}
 !7 = !{!8}