Replace GetNumBytesInBuffer() in MicrosoftCXXNameMangler::mangleArgumentType by tell().

Using GetNumBytesInBuffer() assumes that the stream was not flushed between
the GetNumBytesInBuffer() calls, which may happen to be true or not,
depending on stream policy. tell() always reports the correct stream location.

Do note there are only two more uses of GetNumBytesInBuffer() in LLVM+clang, in
lib/MC/MCAsmStreamer.cpp and lib/Target/R600/InstPrinter/AMDGPUInstPrinter.cpp.
The former may be replacable by tell (needs testing) but while the later can
not be immediatly replaced by tell() as it uses the absolute value of
GetNumBytesInBuffer() rather than the real stream position. Both uses seems
to depend upon flush policy and thus may not work correctly depending upon the
stream behaviour.

Going forward, GetNumBytesInBuffer() should probably be protected, non-accessible
to raw_ostream clients.

llvm-svn: 236389