Delete CC_Default and use the target default CC everywhere

Summary:
Makes functions with implicit calling convention compatible with
function types with a matching explicit calling convention.  This fixes
things like calls to qsort(), which has an explicit __cdecl attribute on
the comparator in Windows headers.

Clang will now infer the calling convention from the declarator.  There
are two cases when the CC must be adjusted during redeclaration:
1. When defining a non-inline static method.
2. When redeclaring a function with an implicit or mismatched
convention.

Fixes PR13457, and allows clang to compile CommandLine.cpp for the
Microsoft C++ ABI.

Excellent test cases provided by Alexander Zinenko!

Reviewers: rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D1231

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189412 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index a17a903..c00ae69 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -728,7 +728,7 @@
     // function to have, if it were to match the name given.
     // FIXME: Calling convention!
     FunctionProtoType::ExtProtoInfo EPI = ConvProto->getExtProtoInfo();
-    EPI.ExtInfo = EPI.ExtInfo.withCallingConv(CC_Default);
+    EPI.ExtInfo = EPI.ExtInfo.withCallingConv(CC_C);
     EPI.ExceptionSpecType = EST_None;
     EPI.NumExceptions = 0;
     QualType ExpectedType