Switch lto codegen to using diagnostic handlers.

Switch lto codegen to using diagnostic handlers.

This patch removes the std::string& argument from a number of C++ LTO API calls
and instead makes them use the installed diagnostic handler. This would also
improve consistency of diagnostic handling infrastructure: if an LTO client used
lto_codegen_set_diagnostic_handler() to install a custom error handler, we do
not want some error messages to go through the custom error handler, and some
other error messages to go into sLastErrorString.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253367 91177308-0d34-0410-b5e6-96231b3b80d8