[Demangle] Add prepend functionality to OutputString

Implement the functionallity of prepend, required by D demangler.
Please read discussion https://reviews.llvm.org/D111414 for context.
See also https://reviews.llvm.org/D111947 .

Reviewed By: dblaikie, Geod24

Differential Revision: https://reviews.llvm.org/D111948

libcxxabi/src/demangle/Utility.h

@@ -95,6 +95,17 @@ public:
 
   OutputBuffer &operator<<(StringView R) { return (*this += R); }
 
+  OutputBuffer prepend(StringView R) {
+    size_t Size = R.size();
+
+    grow(Size);
+    std::memmove(Buffer + Size, Buffer, CurrentPosition);
+    std::memcpy(Buffer, R.begin(), Size);
+    CurrentPosition += Size;
+
+    return *this;
+  }
+
   OutputBuffer &operator<<(char C) { return (*this += C); }
 
   OutputBuffer &operator<<(long long N) {

llvm/include/llvm/Demangle/Utility.h

@@ -95,6 +95,17 @@ public:
 
   OutputBuffer &operator<<(StringView R) { return (*this += R); }
 
+  OutputBuffer prepend(StringView R) {
+    size_t Size = R.size();
+
+    grow(Size);
+    std::memmove(Buffer + Size, Buffer, CurrentPosition);
+    std::memcpy(Buffer, R.begin(), Size);
+    CurrentPosition += Size;
+
+    return *this;
+  }
+
   OutputBuffer &operator<<(char C) { return (*this += C); }
 
   OutputBuffer &operator<<(long long N) {

llvm/unittests/Demangle/OutputBufferTest.cpp

@@ -60,3 +60,21 @@ TEST(OutputBufferTest, Insert) {
 
   std::free(OB.getBuffer());
 }
+
+TEST(OutputBufferTest, Prepend) {
+  OutputBuffer OB;
+
+  OB.prepend("n");
+  EXPECT_EQ("n", toString(OB));
+
+  OB << "abc";
+  OB.prepend("def");
+  EXPECT_EQ("defnabc", toString(OB));
+
+  OB.setCurrentPosition(3);
+
+  OB.prepend("abc");
+  EXPECT_EQ("abcdef", toString(OB));
+
+  std::free(OB.getBuffer());
+}