[PECOFF] Rename link.exe -> lld-link.exe.

Creating the file "link.exe" made some confusion, so it's better to
name it lld-link.exe, as we did for CL (clang-cl.exe).
InstrProf: Actually detect bad headers

<rdar://problem/15950346>
InstrProf: Detect magic numbers in a more scalable way

No functionality change.

<rdar://problem/15950346>
InstrProf: Use move semantics with unique_ptr

<rdar://problem/15950346>
InstrProf: Change magic number to have non-text characters

Include non-text characters in the magic number so that text files can't
match.

<rdar://problem/15950346>
InstrProf: Change magic number to have non-text characters

Include non-text characters in the magic number so that text files can't
match.

<rdar://problem/15950346>