[NativeSession] Implement NativeSession::findSymbolByAddress.

[NativeSession] Implement NativeSession::findSymbolByAddress.

Summary: This implements searching for function symbols and public symbols by address.

More specifically,
-Implements NativeSession::findSymbolByAddress for function symbols and
public symbols. I think data symbols are also searched for, but isn't
implemented in this patch.
-Adds classes for NativeFunctionSymbol and NativePublicSymbol
-Adds a '-use-native-pdb-reader' option to llvm-symbolizer, for testing
purposes.

Reviewers: rnk, amccarth, labath

Subscribers: mgorny, hiraditya, MaskRay, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79269