WebAssembly: allow swiftcall attribute

This removes the warning when attempting to compile a swiftcall function.

I'm not sure why https://reviews.llvm.org/D19414 didn't add this...

Author: zhuowei

lib/Basic/Targets/WebAssembly.h

@@ -114,6 +114,16 @@ class LLVM_LIBRARY_VISIBILITY WebAssemblyTargetInfo : public TargetInfo {
                ? (IsSigned ? SignedLongLong : UnsignedLongLong)
                : TargetInfo::getLeastIntTypeByWidth(BitWidth, IsSigned);
   }
+
+  CallingConvCheckResult checkCallingConvention(CallingConv CC) const override {
+    switch (CC) {
+    case CC_C:
+    case CC_Swift:
+      return CCCR_OK;
+    default:
+      return CCCR_Warning;
+    }
+  }
 };
 class LLVM_LIBRARY_VISIBILITY WebAssembly32TargetInfo
     : public WebAssemblyTargetInfo {