Fix SelectionDAG Graph Printing on Windows

Currently, when compiling to IR (presumably at the clang level) LLVM
mangles symbols and sometimes they have illegal file characters
including `?`'s in them. This causes a problem when building a graph via
llc on Windows because the code currently passes the machine function
name all the way down to the Windows API which frequently returns error
123  **ERROR_INVALID_NAME**
https://docs.microsoft.com/en-us/windows/win32/debug/system-error-codes--0-499-

Thus, we need to remove those illegal characters from the machine
function name before generating a graph, which is the purpose of this
patch.
https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file

I've created a static helper function replace_illegal_filename_chars
which within GraphWriter.cpp to help with replacing illegal file
character names before generating a dot graph filename.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D76863