Debug Info: set default to gdwarf-2 for Darwin.

Darwin systems currently do not support dwarf version 3 or above. When we are
ready, we can bump the default to gdwarf-4 for Darwin.

For other systems, the default is dwarf version 3, if everything goes smoothly,
we can bump the version to 4.

rdar://13591116


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185483 91177308-0d34-0410-b5e6-96231b3b80d8
Look for corrections in enclosing namespaces that require a global NestedNameSpecifier.

CorrectTypo will now see and consider those corrections that are effectively
shadowed by other declarations in a closer context when resolved via an
unqualified lookup. This involves adding any parent namespaces to the set of
namespaces as fully-qualified name specifiers, and also adding potential
corrections that passed name lookup but were rejected by the given
CorrectionCandidateCallback into the set of failed corrections that should be
tried with the set of namespace specifiers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185486 91177308-0d34-0410-b5e6-96231b3b80d8
Allow typo correction to try removing nested name specifiers.

The removal is tried by retrying the failed lookup of a correction
candidate with either the MemberContext or SS (CXXScopeSpecifier) or
both set to NULL if they weren't already. If the candidate identifier
is then looked up successfully, make a note in the candidate that the
SourceRange should include any existing nested name specifier even if
the candidate isn't adding a different one (i.e. the candidate has a
NULL NestedNameSpecifier).

Also tweak the diagnostic messages to differentiate between a suggestion
that just replaces the identifer but leaves the existing nested name
specifier intact and one that replaces the entire qualified identifier,
in cases where the suggested replacement is unqualified.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185487 91177308-0d34-0410-b5e6-96231b3b80d8