Fix stripping of _ when looking for symbols in IRExecutionUnit.

Previously we would try both versions of a symbol -- the one with _ in it and
the one without -- in all cases, because we didn't know what the current
platform's policy was.  However, stripping _ is only necessary on platforms
where _ is the prefix for global symbols.

There's an API that does this, though, on llvm::DataLayout, so this patch fixes
IRExecutionUnit to use that API to determine whether or not to strip _ from the
symbol or not.

llvm-svn: 260767