OutputStream - fix static analyzer warnings. NFCI.

 - uninitialized variables
 - make getBufferCapacity() const

llvm/include/llvm/Demangle/Utility.h

@@ -25,9 +25,9 @@ DEMANGLE_NAMESPACE_BEGIN
 // Stream that AST nodes write their string representation into after the AST
 // has been parsed.
 class OutputStream {
-  char *Buffer;
-  size_t CurrentPosition;
-  size_t BufferCapacity;
+  char *Buffer = nullptr;
+  size_t CurrentPosition = 0;
+  size_t BufferCapacity = 0;
 
   // Ensure there is at least n more positions in buffer.
   void grow(size_t N) {
@@ -137,7 +137,7 @@ class OutputStream {
 
   char *getBuffer() { return Buffer; }
   char *getBufferEnd() { return Buffer + CurrentPosition - 1; }
-  size_t getBufferCapacity() { return BufferCapacity; }
+  size_t getBufferCapacity() const { return BufferCapacity; }
 };
 
 template <class T> class SwapAndRestore {