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/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 84f8a34..f4c4226 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -3094,9 +3094,7 @@
   }
 
   virtual CallingConvCheckResult checkCallingConvention(CallingConv CC) const {
-    return (CC == CC_Default ||
-            CC == CC_C || 
-            CC == CC_IntelOclBicc) ? CCCR_OK : CCCR_Warning;
+    return (CC == CC_C || CC == CC_IntelOclBicc) ? CCCR_OK : CCCR_Warning;
   }
 
   virtual CallingConv getDefaultCallingConv(CallingConvMethodType MT) const {