Target/X86: Add explicit Win64 and System V/x86-64 calling conventions.

Summary:
This patch adds explicit calling convention types for the Win64 and
System V/x86-64 ABIs. This allows code to override the default, and use
the Win64 convention on a target that wants to use SysV (and
vice-versa). This is needed to implement the `ms_abi` and `sysv_abi` GNU
attributes.

Reviewers:

CC:

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186144 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AsmParser/LLParser.cpp b/lib/AsmParser/LLParser.cpp
index 941a371..62a07f5 100644
--- a/lib/AsmParser/LLParser.cpp
+++ b/lib/AsmParser/LLParser.cpp
@@ -1344,6 +1344,8 @@
 ///   ::= 'ptx_device'
 ///   ::= 'spir_func'
 ///   ::= 'spir_kernel'
+///   ::= 'x86_64_sysvcc'
+///   ::= 'x86_64_win64cc'
 ///   ::= 'cc' UINT
 ///
 bool LLParser::ParseOptionalCallingConv(CallingConv::ID &CC) {
@@ -1364,6 +1366,8 @@
   case lltok::kw_spir_kernel:    CC = CallingConv::SPIR_KERNEL; break;
   case lltok::kw_spir_func:      CC = CallingConv::SPIR_FUNC; break;
   case lltok::kw_intel_ocl_bicc: CC = CallingConv::Intel_OCL_BI; break;
+  case lltok::kw_x86_64_sysvcc:  CC = CallingConv::X86_64_SysV; break;
+  case lltok::kw_x86_64_win64cc: CC = CallingConv::X86_64_Win64; break;
   case lltok::kw_cc: {
       unsigned ArbitraryCC;
       Lex.Lex();