Add driver arguments -ftemplate-depth=N and -fconstexpr-depth=N, with the same

semantics and defaults as the corresponding g++ arguments. The historical g++
argument -ftemplate-depth-N is kept for compatibility, but modern g++ versions
no longer document that option.

Add -cc1 argument -fconstexpr-depth N to implement the corresponding
functionality.

The -ftemplate-depth=N part of this fixes PR9890.

llvm-svn: 145045