Warn about code that uses variables and functions with internal linkage

without defining them.  This should be an error, but I'm paranoid about
"uses" that end up not actually requiring a definition.  I'll revisit later.

Also, teach IR generation to not set internal linkage on variable
declarations, just for safety's sake.  Doing so produces an invalid module
if the variable is not ultimately defined.

Also, fix several places in the test suite where we were using internal
functions without definitions.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126016 91177308-0d34-0410-b5e6-96231b3b80d8