Change the diagnostics interface to take an array of pointers to

Change the diagnostics interface to take an array of pointers to
strings instead of array of strings.  This reduces string copying
in some not-very-important cases, but paves the way for future
improvements.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59494 91177308-0d34-0410-b5e6-96231b3b80d8