[Basic] Ensure empty NullTerminatedStringRef have valid pointer

NullterminatedStringRef is meant to be safe for reading 'data()' as a
valid C-string.

rdar://108215490

Author: rintaro

include/swift/Basic/StringExtras.h

@@ -490,7 +490,7 @@ class NullTerminatedStringRef {
 
   /// Create a null-terminated string, copying \p Str into \p A .
   template <typename Allocator>
-  NullTerminatedStringRef(StringRef Str, Allocator &A) : Ref() {
+  NullTerminatedStringRef(StringRef Str, Allocator &A) : Ref("") {
     if (Str.empty())
       return;
 

lib/Sema/TypeCheckMacros.cpp

@@ -677,6 +677,7 @@ Expr *swift::expandMacroExpr(
     return nullptr;
 
   // Evaluate the macro.
+  llvm::BumpPtrAllocator allocator;
   NullTerminatedStringRef evaluatedSource;
 
   MacroDecl *macro = cast<MacroDecl>(macroRef.getDecl());
@@ -722,7 +723,6 @@ Expr *swift::expandMacroExpr(
     // Expand the definition with the given arguments.
     auto result = expandMacroDefinition(
         macroDef.getExpanded(), macro, expr->getArgs());
-    llvm::MallocAllocator allocator;
     evaluatedSource = NullTerminatedStringRef(result, allocator);
     break;
   }
@@ -866,6 +866,7 @@ swift::expandFreestandingMacro(MacroExpansionDecl *med) {
     return None;
 
   // Evaluate the macro.
+  llvm::BumpPtrAllocator allocator;
   NullTerminatedStringRef evaluatedSource;
 
   MacroDecl *macro = cast<MacroDecl>(med->getMacroRef().getDecl());
@@ -899,7 +900,6 @@ swift::expandFreestandingMacro(MacroExpansionDecl *med) {
     // Expand the definition with the given arguments.
     auto result = expandMacroDefinition(
         macroDef.getExpanded(), macro, med->getArgs());
-    llvm::MallocAllocator allocator;
     evaluatedSource = NullTerminatedStringRef(result, allocator);
     break;
   }
@@ -1092,6 +1092,7 @@ evaluateAttachedMacro(MacroDecl *macro, Decl *attachedTo, CustomAttr *attr,
   }
 
   // Evaluate the macro.
+  llvm::BumpPtrAllocator allocator;
   NullTerminatedStringRef evaluatedSource;
 
   std::string discriminator;
@@ -1114,7 +1115,6 @@ evaluateAttachedMacro(MacroDecl *macro, Decl *attachedTo, CustomAttr *attr,
     // Expand the definition with the given arguments.
     auto result = expandMacroDefinition(
         macroDef.getExpanded(), macro, attr->getArgs());
-    llvm::MallocAllocator allocator;
     evaluatedSource = NullTerminatedStringRef(result, allocator);
     break;
   }