Revert "Don't create a SymbolTable in Function when the LLVMContext discards value names (NFC)"

This reverts commit r281806. It introduces undefined behavior as an
API is returning a reference to the Symtab

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281808 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed