[MCJIT] Teach RuntimeDyldChecker to handle underscores at the start of symbols.

RuntimeDyldChecker had been testing isalpha(Expr[0]) to recognise symbol tokens,
and throwing unrecognized token errors when it hit symbols with leading
underscores. This fixes that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213706 91177308-0d34-0410-b5e6-96231b3b80d8