[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/unittests/DebugInfo/PDB/PDBApiTest.cpp b/llvm/unittests/DebugInfo/PDB/PDBApiTest.cpp
index 41b6798..f43404f 100644
--- a/llvm/unittests/DebugInfo/PDB/PDBApiTest.cpp
+++ b/llvm/unittests/DebugInfo/PDB/PDBApiTest.cpp
@@ -68,7 +68,7 @@
   uint64_t getLoadAddress() const override { return 0; }
   bool setLoadAddress(uint64_t Address) override { return false; }
   std::unique_ptr<PDBSymbolExe> getGlobalScope() override { return nullptr; }
-  std::unique_ptr<PDBSymbol> getSymbolById(uint32_t SymbolId) const override {
+  std::unique_ptr<PDBSymbol> getSymbolById(SymIndexId SymbolId) const override {
     return nullptr;
   }
   std::unique_ptr<IPDBSourceFile>