[BPF] Fix linking issues in static map initializers (#91310)

When BPF object files are linked with bpftool, every symbol must be
accompanied by BTF info. Ensure that extern functions referenced by
global variable initializers are included in BTF.

The primary motivation is "static" initialization of PROG maps:

```c
extern int elsewhere(struct xdp_md *);

struct {
  __uint(type, BPF_MAP_TYPE_PROG_ARRAY);
  __uint(max_entries, 1);
  __type(key, int);
  __type(value, int);
  __array(values, int (struct xdp_md *));
} prog_map SEC(".maps") = { .values = { elsewhere } };
```

BPF backend needs debug info to produce BTF. Debug info is not
normally generated for external variables and functions. Previously, it
was solved differently for variables (collecting variable declarations
    in ExternalDeclarations vector) and functions (logic invoked during
    codegen in CGExpr.cpp).

This patch generalises ExternalDefclarations to include both function
and variable declarations. This change ensures that function references
    are not missed no matter the context. Previously external functions
    referenced in constant expressions lacked debug info.

1 files changed, 2 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.h b/clang/lib/CodeGen/CodeGenModule.h
index 4796d42..0444f9f 100644
--- a/clang/lib/CodeGen/CodeGenModule.h
+++ b/clang/lib/CodeGen/CodeGenModule.h
@@ -1338,7 +1338,7 @@ public:
 
   void EmitTentativeDefinition(const VarDecl *D);
 
-  void EmitExternalDeclaration(const VarDecl *D);
+  void EmitExternalDeclaration(const DeclaratorDecl *D);
 
   void EmitVTable(CXXRecordDecl *Class);
 
@@ -1690,6 +1690,7 @@ private:
 
   void EmitGlobalVarDefinition(const VarDecl *D, bool IsTentative = false);
   void EmitExternalVarDeclaration(const VarDecl *D);
+  void EmitExternalFunctionDeclaration(const FunctionDecl *D);
   void EmitAliasDefinition(GlobalDecl GD);
   void emitIFuncDefinition(GlobalDecl GD);
   void emitCPUDispatchDefinition(GlobalDecl GD);