[CGDebugInfo] Emit subprograms for decls when AT_tail_call is understood (reland with fixes)

Currently, clang emits subprograms for declared functions when the
target debugger or DWARF standard is known to support entry values
(DW_OP_entry_value & the GNU equivalent).

Treat DW_AT_tail_call the same way to allow debuggers to follow cross-TU
tail calls.

Pre-patch debug session with a cross-TU tail call:

```
  * frame #0: 0x0000000100000fa4 main`target at b.c:4:3 [opt]
    frame #1: 0x0000000100000f99 main`main at a.c:8:10 [opt]
```

Post-patch (note that the tail-calling frame, "helper", is visible):

```
  * frame #0: 0x0000000100000fa4 main`target at b.c:4:3 [opt]
    frame #1: 0x0000000100000f80 main`helper [opt] [artificial]
    frame #2: 0x0000000100000f99 main`main at a.c:8:10 [opt]
```

This was reverted in 5b9a072c because it attached declaration
subprograms to inlinable builtin calls, which interacted badly with the
MergeICmps pass. The fix is to not attach declarations to builtins.

rdar://46577651

Differential Revision: https://reviews.llvm.org/D69743

1 files changed, 2 insertions, 14 deletions

diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index ade6e46d1bc..81eed4330cd 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -692,18 +692,6 @@ getRequiredQualification(ASTContext &Context, const DeclContext *CurContext,
   return Result;
 }
 
-/// Determine whether \p Id is a name reserved for the implementation (C99
-/// 7.1.3, C++ [lib.global.names]).
-static bool isReservedName(const IdentifierInfo *Id,
-                           bool doubleUnderscoreOnly = false) {
-  if (Id->getLength() < 2)
-    return false;
-  const char *Name = Id->getNameStart();
-  return Name[0] == '_' &&
-         (Name[1] == '_' ||
-          (Name[1] >= 'A' && Name[1] <= 'Z' && !doubleUnderscoreOnly));
-}
-
 // Some declarations have reserved names that we don't want to ever show.
 // Filter out names reserved for the implementation if they come from a
 // system header.
@@ -713,13 +701,13 @@ static bool shouldIgnoreDueToReservedName(const NamedDecl *ND, Sema &SemaRef) {
     return false;
 
   // Ignore reserved names for compiler provided decls.
-  if (isReservedName(Id) && ND->getLocation().isInvalid())
+  if (Id->isReservedName() && ND->getLocation().isInvalid())
     return true;
 
   // For system headers ignore only double-underscore names.
   // This allows for system headers providing private symbols with a single
   // underscore.
-  if (isReservedName(Id, /*doubleUnderscoreOnly=*/true) &&
+  if (Id->isReservedName(/*doubleUnderscoreOnly=*/true) &&
       SemaRef.SourceMgr.isInSystemHeader(
           SemaRef.SourceMgr.getSpellingLoc(ND->getLocation())))
     return true;