Make codeview::StringTable.

Previously we had knowledge of how to serialize and deserialize
a string table inside of DebugInfo/PDB, but the string table
that it serializes contains a piece that is actually considered
CodeView and can appear outside of a PDB.  We already have logic
in llvm-readobj and MCCodeView to read and write this format,
so it doesn't make sense to duplicate the logic in DebugInfoPDB
as well.

This patch makes codeview::StringTable (for writing) and
codeview::StringTableRef (for reading), updates DebugInfoPDB
to use these classes for its own writing, and updates llvm-readobj
to additionally use StringTableRef for reading.

It's a bit more difficult to get MCCodeView to use this for
writing, but it's a logical next step.

llvm-svn: 301986
diff --git a/llvm/lib/DebugInfo/PDB/Native/DbiStream.cpp b/llvm/lib/DebugInfo/PDB/Native/DbiStream.cpp
index 4802cc6..db70380 100644
--- a/llvm/lib/DebugInfo/PDB/Native/DbiStream.cpp
+++ b/llvm/lib/DebugInfo/PDB/Native/DbiStream.cpp
@@ -146,7 +146,7 @@
 
   if (ECSubstream.getLength() > 0) {
     BinaryStreamReader ECReader(ECSubstream);
-    if (auto EC = ECNames.load(ECReader))
+    if (auto EC = ECNames.reload(ECReader))
       return EC;
   }