Add a note that points to the linkage specifier for the C++ linkage errors

This commit improves the "must have C++ linkage" error diagnostics that are
emitted for C++ declarations like templates and literal operators by adding an
additional note that points to the appropriate extern "C" linkage specifier.

rdar://19021120

Differential Revision: https://reviews.llvm.org/D26189

llvm-svn: 285823