tools: teach objdump about FILE aux records

Add support for file auxiliary symbol entries in COFF symbol tables.  A COFF
symbol table with a FILE entry is followed by sizeof(__FILE__) / 18 auxiliary
symbol records which contain the filename.  Read them and form the original
filename that the record contains.  Then display the name in the output.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206126 91177308-0d34-0410-b5e6-96231b3b80d8