Solidify the assumption that a DW_TAG_subprogram's type is a DW_TAG_subroutine_type

There were bits & pieces of code lying around that may've given the
impression that debug info metadata supported the possibility that a
subprogram's type could be specified by a non-subroutine type describing
the return type of a void function. This support was incomplete &
unnecessary. Asserts & API have been changed to make the desired usage
more clear.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182532 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/IR/DIBuilder.cpp b/lib/IR/DIBuilder.cpp
index 1c79945..8edc6a0 100644
--- a/lib/IR/DIBuilder.cpp
+++ b/lib/IR/DIBuilder.cpp
@@ -969,13 +969,15 @@
                                        StringRef Name,
                                        StringRef LinkageName,
                                        DIFile File, unsigned LineNo,
-                                       DIType Ty,
+                                       DICompositeType Ty,
                                        bool isLocalToUnit, bool isDefinition,
                                        unsigned ScopeLine,
                                        unsigned Flags, bool isOptimized,
                                        Function *Fn,
                                        MDNode *TParams,
                                        MDNode *Decl) {
+  assert(Ty.getTag() == dwarf::DW_TAG_subroutine_type &&
+         "function types should be subroutines");
   Value *TElts[] = { GetTagConstant(VMContext, DW_TAG_base_type) };
   Value *Elts[] = {
     GetTagConstant(VMContext, dwarf::DW_TAG_subprogram),
@@ -1014,7 +1016,7 @@
                                      StringRef Name,
                                      StringRef LinkageName,
                                      DIFile F,
-                                     unsigned LineNo, DIType Ty,
+                                     unsigned LineNo, DICompositeType Ty,
                                      bool isLocalToUnit,
                                      bool isDefinition,
                                      unsigned VK, unsigned VIndex,
@@ -1023,6 +1025,8 @@
                                      bool isOptimized,
                                      Function *Fn,
                                      MDNode *TParam) {
+  assert(Ty.getTag() == dwarf::DW_TAG_subroutine_type &&
+         "function types should be subroutines");
   Value *TElts[] = { GetTagConstant(VMContext, DW_TAG_base_type) };
   Value *Elts[] = {
     GetTagConstant(VMContext, dwarf::DW_TAG_subprogram),