Document the -fno-limit-debug-info switch in the man page.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197819 91177308-0d34-0410-b5e6-96231b3b80d8
Remove an outdated statement about debugging optimized code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197820 91177308-0d34-0410-b5e6-96231b3b80d8