Introduce CompilerInvocation::setLangDefaults function

This patch refactors the CompilerInvocation code to introduce a
CompilerInvocation::setLangDefaults function, which can set up a
LangOptions with the defaults for a given language and language
standard.  This function is useful for non-command line based Clang
clients which need to set up a CompilerInvocation manually for a
specific language.

llvm-svn: 120874