[Backtracing][IRGen] Add a semantic attribute to force frame pointer.

The Swift backtracer's frame pointer unwinder cannot work on Linux
without this change, because the compiler omits the frame pointer from
the function in libSwift_Backtracing that actually captures the stack.

rdar://110260855

Author: al45tair

include/swift/AST/SemanticAttrs.def

@@ -144,5 +144,8 @@ SEMANTICS_ATTR(NO_PERFORMANCE_ANALYSIS, "no_performance_analysis")
 // that may cause the user to think there is a bug in the compiler.
 SEMANTICS_ATTR(NO_MOVEONLY_DIAGNOSTICS, "sil.optimizer.moveonly.diagnostic.ignore")
 
+// Force the use of the frame pointer for the specified function
+SEMANTICS_ATTR(USE_FRAME_POINTER, "use_frame_pointer")
+
 #undef SEMANTICS_ATTR
 

lib/IRGen/IRGenSIL.cpp

@@ -21,6 +21,7 @@
 #include "swift/AST/IRGenOptions.h"
 #include "swift/AST/ParameterList.h"
 #include "swift/AST/Pattern.h"
+#include "swift/AST/SemanticAttrs.h"
 #include "swift/AST/SubstitutionMap.h"
 #include "swift/AST/Types.h"
 #include "swift/Basic/ExternalUnion.h"
@@ -1835,6 +1836,11 @@ IRGenSILFunction::IRGenSILFunction(IRGenModule &IGM, SILFunction *f)
     }
   }
 
+  // If we have @_semantics("use_frame_pointer"), force the use of a
+  // frame pointer for this function.
+  if (f->hasSemanticsAttr(semantics::USE_FRAME_POINTER))
+    CurFn->addFnAttr("frame-pointer", "all");
+
   // Disable inlining of coroutine functions until we split.
   if (f->getLoweredFunctionType()->isCoroutine()) {
     CurFn->addFnAttr(llvm::Attribute::NoInline);