Unpluralize -Wfoo-conversions warnings for consistency.

Err on the side of brevity and rename (while providing aliases for the original
name) -Wbool-conversions, -Wint-conversions, and -Wvector-conversions for
consistency with constant, literal, string, and sign conversion warnings. And
name the diagnostic groups explicitly while I'm here rather than rewriting the
string in the groups and sema td files.

Curiously, vector-conversion is not under -Wconversion. Perhaps it should be.

llvm-svn: 152776