[llvm] Forward-declare formatted_raw_ostream (NFC)

Various *TargetStreamer.h need formatted_raw_ostream but rely on a
forward declaration of formatted_raw_ostream in MCStreamer.h.  This
patch adds forward declarations right in *TargetStreamer.h.

While we are at it, this patch removes the one in MCStreamer.h, where
it is unnecessary.

include/llvm/MC/MCStreamer.h

@@ -39,7 +39,6 @@
 namespace llvm {
 
 class AssemblerConstantPools;
-class formatted_raw_ostream;
 class MCAsmBackend;
 class MCCodeEmitter;
 class MCContext;

lib/Target/AMDGPU/MCTargetDesc/AMDGPUTargetStreamer.h

@@ -23,6 +23,7 @@ class MCSymbol;
 class MDNode;
 class Module;
 class Type;
+class formatted_raw_ostream;
 
 namespace AMDGPU {
 namespace HSAMD {

lib/Target/Mips/MipsTargetStreamer.h

@@ -19,6 +19,8 @@
 
 namespace llvm {
 
+class formatted_raw_ostream;
+
 class MipsTargetStreamer : public MCTargetStreamer {
 public:
   MipsTargetStreamer(MCStreamer &S);

lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.h

@@ -14,6 +14,8 @@
 
 namespace llvm {
 
+class formatted_raw_ostream;
+
 class RISCVTargetStreamer : public MCTargetStreamer {
 public:
   RISCVTargetStreamer(MCStreamer &S);

lib/Target/Sparc/MCTargetDesc/SparcTargetStreamer.h

@@ -13,6 +13,9 @@
 #include "llvm/MC/MCStreamer.h"
 
 namespace llvm {
+
+class formatted_raw_ostream;
+
 class SparcTargetStreamer : public MCTargetStreamer {
   virtual void anchor();
 

lib/Target/WebAssembly/MCTargetDesc/WebAssemblyTargetStreamer.h

@@ -22,6 +22,7 @@
 namespace llvm {
 
 class MCSymbolWasm;
+class formatted_raw_ostream;
 
 /// WebAssembly-specific streamer interface, to implement support
 /// WebAssembly-specific assembly directives.