[PDB] Serialize records into a stack-allocated buffer.

We were using a std::vector<> and resizing to MaxRecordLength,
which is ~64KB.  We would then do this repeatedly often many
times in a tight loop, which was causing measurable performance
impact when linking PDBs.

Patch by Alex Telishev
Differential Revision: https://reviews.llvm.org/D36940

llvm-svn: 311375
diff --git a/llvm/lib/DebugInfo/CodeView/SymbolSerializer.cpp b/llvm/lib/DebugInfo/CodeView/SymbolSerializer.cpp
index 9a2e776..0071ecc 100644
--- a/llvm/lib/DebugInfo/CodeView/SymbolSerializer.cpp
+++ b/llvm/lib/DebugInfo/CodeView/SymbolSerializer.cpp
@@ -21,8 +21,7 @@
 
 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) {