[Symbolize][PDB] Switch llvm-symbolizer to use PDB_ReaderType::Native.

Since native PDB reading has been implemented for symbolizing,
switch to using the native PDB reader by default, unless
LLVM_ENABLE_DIA_SDK is on.

Bug: https://bugs.llvm.org/show_bug.cgi?id=41795

Differential Revision: https://reviews.llvm.org/D84286