[PDB] Change uint32_t to SymIndex wherever it makes sense.

Although it's just a typedef, it helps for readability.  NFC.

llvm-svn: 341863
diff --git a/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp b/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
index c970323..baab0a2 100644
--- a/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
+++ b/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
@@ -89,7 +89,7 @@
 }
 
 std::unique_ptr<PDBSymbol>
-NativeSession::getSymbolById(uint32_t SymbolId) const {
+NativeSession::getSymbolById(SymIndexId SymbolId) const {
   return Cache.getSymbolById(SymbolId);
 }