[NVPTX] Correct the condition to print "\t}\n" for DWARF sections

`NVPTXTargetStreamer::changeSection` needs `CurSection` to properly
print "\t}\n" for DWARF sections.

db48f1a added
`MCStreamer::changeSection` before `TS->changeSection`,
making `CurSection == Section` when `getCurrentSectionOnly` switches to
`CurFrag` (626eef5).

cf311a1 did fix the bug, but the
mechanism is confusing. Fix it using nullable getCurrentSectionOnly().

Author: MaskRay

llvm/lib/MC/MCAsmStreamer.cpp

@@ -511,13 +511,13 @@ void MCAsmStreamer::emitExplicitComments() {
 }
 
 void MCAsmStreamer::changeSection(MCSection *Section, uint32_t Subsection) {
-  MCStreamer::changeSection(Section, Subsection);
   if (MCTargetStreamer *TS = getTargetStreamer()) {
-    TS->changeSection(getCurrentSectionOnly(), Section, Subsection, OS);
+    TS->changeSection(getCurrentSection().first, Section, Subsection, OS);
   } else {
     Section->printSwitchToSection(*MAI, getContext().getTargetTriple(), OS,
                                   Subsection);
   }
+  MCStreamer::changeSection(Section, Subsection);
 }
 
 void MCAsmStreamer::emitELFSymverDirective(const MCSymbol *OriginalSym,

llvm/lib/Target/NVPTX/MCTargetDesc/NVPTXTargetStreamer.cpp

@@ -88,10 +88,8 @@ void NVPTXTargetStreamer::changeSection(const MCSection *CurSection,
   assert(!SubSection && "SubSection is not null!");
   const MCObjectFileInfo *FI = getStreamer().getContext().getObjectFileInfo();
   // Emit closing brace for DWARF sections only.
-  if (isDwarfSection(FI, CurSection) && HasDWARFSections) {
-    HasDWARFSections = false;
+  if (isDwarfSection(FI, CurSection))
     OS << "\t}\n";
-  }
   if (isDwarfSection(FI, Section)) {
     // Emit DWARF .file directives in the outermost scope.
     outputDwarfFileDirectives();
@@ -102,7 +100,6 @@ void NVPTXTargetStreamer::changeSection(const MCSection *CurSection,
     // DWARF sections are enclosed into braces - emit the open one.
     OS << "\t{\n";
     HasSections = true;
-    HasDWARFSections = true;
   }
 }
 

llvm/lib/Target/NVPTX/MCTargetDesc/NVPTXTargetStreamer.h

@@ -19,7 +19,6 @@ class NVPTXTargetStreamer : public MCTargetStreamer {
 private:
   SmallVector<std::string, 4> DwarfFiles;
   bool HasSections = false;
-  bool HasDWARFSections = false;
 
 public:
   NVPTXTargetStreamer(MCStreamer &S);