Replace std::vector to std::array in SymbolSerializer

[alex.telishev]

Summary

SymbolSerializer::writeOneSymbol function is called very often and creates a temporary SymbolSerializer
RecordBuffer is always created with a constant size so it's much faster to allocate this memory on the stack instead of heap

[zturner]

The results of this and the other 2 patches look promising. Some initial benchmarks where I linked clang.exe using MSVC, lld with patch, and lld without patch are:

MSVC linker - 71.59s
lld without these 3 patches - 47.98s
lld with these 3 patches - 29.91s

Definitely a solid improvement. I'll run some more timings on Monday when I'm back in the office (as well as looking more closely at the 3 patches).

Thanks!

llvm/trunk/include/llvm/DebugInfo/CodeView/SymbolSerializer.h

 #include <cstdint>
 #include <vector>
 
 namespace llvm {
 namespace codeview {
 
 class SymbolSerializer : public SymbolVisitorCallbacks {
   BumpPtrAllocator &Storage;
-  std::vector<uint8_t> RecordBuffer;
+  // Since this is a fixed size buffer, use a stack allocated buffer.  This
+  // yields measurable performance increase over the repeated heap allocations
+  // when serializing many independent records via writeOneSymbol.
+  std::array<uint8_t, MaxRecordLength> RecordBuffer;
   MutableBinaryByteStream Stream;
   BinaryStreamWriter Writer;
   SymbolRecordMapping Mapping;
   Optional<SymbolKind> CurrentSymbol;
 
   Error writeRecordPrefix(SymbolKind Kind) {
     RecordPrefix Prefix;
     Prefix.RecordKind = Kind;

llvm/trunk/lib/DebugInfo/CodeView/SymbolSerializer.cpp

 #include <cstdint>
 #include <cstring>
 
 using namespace llvm;
 using namespace llvm::codeview;
 
 SymbolSerializer::SymbolSerializer(BumpPtrAllocator &Allocator,
                                    CodeViewContainer Container)
-    : Storage(Allocator), RecordBuffer(MaxRecordLength),
-      Stream(RecordBuffer, support::little), Writer(Stream),
+    : Storage(Allocator), Stream(RecordBuffer, support::little), Writer(Stream),
       Mapping(Writer, Container) {}
 
 Error SymbolSerializer::visitSymbolBegin(CVSymbol &Record) {
   assert(!CurrentSymbol.hasValue() && "Already in a symbol mapping!");
 
   Writer.setOffset(0);
 
   if (auto EC = writeRecordPrefix(Record.kind()))