clang-cl: parse (but don't support) the /o flag

Apparently folks run into this (PR17097). The flag is not supported by
MSVC either, but we should parse it so we don't get confused when it occurs.

This changes the clang-cl output for "clang-cl /c /o foo.obj" from:

  clang-cl.exe: error: no such file or directory: '/o'
  clang-cl.exe: error: no such file or directory: 'foo.obj'

to:

  clang-cl.exe: warning: argument unused during compilation: '/o bajs.obj'
[docs] Some typographical fixes.
Debug Info: Move isSubprogramContext from DebugInfo to DwarfDebug.

This helper function needs the type identifier map when we switch
DIType::getContext to return DIScopeRef instead of DIScope.

Since isSubprogramContext is used by DwarfDebug only, We move it to DwarfDebug
to have easy access to the map.