[lldb][PDB] Rename GetDeclarationForSymbol() -> AddSourceInfoToDecl()

The old name of this function was confusing for me, when I started working on the PDB parser. The new name clearifies that the function adds file, line and column (typically referred as source info) and indicates that the information is stored in the provided decl parameter.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D146286