Emit DW_TAG_formal_parameter for unnamed parameters.

This change essentially reverts r87069 which came without a test case. It
causes no regressions in the GDB 7.5 test suite & fixes 25 xfails (commit
to the test suite to follow). If anyone can present a test case that
demonstrates why this check is necessary I'd be happy to account for it in one
way or another.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171609 91177308-0d34-0410-b5e6-96231b3b80d8