[RuntimeDyld] Improve error diagnostic in RuntimeDyldChecker.

The compiler often emits assembler-local labels (beginning with 'L') for use in
relocation expressions, however these aren't included in the object files.
Teach RuntimeDyldChecker to warn the user if they try to use one of these in an
expression, since it will never work.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212777 91177308-0d34-0410-b5e6-96231b3b80d8