Move SymbolTable::addFile to InputFiles.cpp.

The symbol table used to be a container of vectors of input files,
but that's no longer the case because the vectors are moved out of
SymbolTable and are now global variables.

Therefore, addFile doesn't have to belong to any class. This patch
moves the function out of the class.

This patch is a preparation for my RFC [1].

[1] http://lists.llvm.org/pipermail/llvm-dev/2019-April/131902.html

Differential Revision: https://reviews.llvm.org/D61854

llvm-svn: 360666