[Bitcode] Add enums for call instruction markers and flags. NFC.

This commit adds enums in LLVMBitCodes.h to improve readability and
maintainability. This is a follow-up to r252368 which was discussed
here:

http://reviews.llvm.org/D12923

llvm-svn: 252395
diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 7497535..b5b79b2 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -2130,9 +2130,11 @@
     Code = bitc::FUNC_CODE_INST_CALL;
 
     Vals.push_back(VE.getAttributeID(CI.getAttributes()));
-    Vals.push_back((CI.getCallingConv() << 1) | unsigned(CI.isTailCall()) |
-                   unsigned(CI.isMustTailCall()) << 14 | 1 << 15 |
-                   unsigned(CI.isNoTailCall()) << 16);
+    Vals.push_back(CI.getCallingConv() << bitc::CALL_CCONV |
+                   unsigned(CI.isTailCall()) << bitc::CALL_TAIL |
+                   unsigned(CI.isMustTailCall()) << bitc::CALL_MUSTTAIL |
+                   1 << bitc::CALL_EXPLICIT_TYPE |
+                   unsigned(CI.isNoTailCall()) << bitc::CALL_NOTAIL);
     Vals.push_back(VE.getTypeID(FTy));
     PushValueAndType(CI.getCalledValue(), InstID, Vals, VE);  // Callee