[Clang][ARM] __va_list declaration is not saved in ASTContext causing compilation error or crash

Summary:
When the code is compiled for arm32 and the builtin `__va_list` declaration is created by `CreateAAPCSABIBuiltinVaListDecl`, the declaration is not saved in the `ASTContext` which may lead to a compilation error or crash.

Minimal reproducer I was able to find:
**header.h**
```
#include <stdarg.h>
typedef va_list va_list_1;
```

**test.cpp**
```
typedef __builtin_va_list va_list_2;
void foo(const char* format, ...) { va_list args; va_start( args, format ); }
```

Steps to reproduce:
```
clang -x c++-header --target=armv7l-linux-eabihf header.h
clang -c -include header.h --target=armv7l-linux-eabihf test.cpp
```

Compilation error:
```
error: non-const lvalue reference to type '__builtin_va_list'
      cannot bind to a value of unrelated type 'va_list' (aka '__builtin_va_list')
```

Compiling the same code as a C source leads to a crash:
```
clang --target=armv7l-linux-eabihf header.h
clang -c -x c -include header.h --target=armv7l-linux-eabihf test.cpp
```

Reviewers: logan, rsmith

Subscribers: cfe-commits, asl, aemerson, rengolin

Differential Revision: http://reviews.llvm.org/D18557

llvm-svn: 264930

clang/lib/AST/ASTContext.cpp

@@ -6389,6 +6389,7 @@ CreateAAPCSABIBuiltinVaListDecl(const ASTContext *Context) {
 
   // };
   VaListDecl->completeDefinition();
+  Context->VaListTagDecl = VaListDecl;
 
   // typedef struct __va_list __builtin_va_list;
   QualType T = Context->getRecordType(VaListDecl);

clang/test/PCH/Inputs/__va_list_tag-typedef.h

@@ -0,0 +1,4 @@
+// Header for PCH test __va_list_tag-typedef.c
+
+#include <stdarg.h>
+typedef va_list va_list_1;

clang/test/PCH/__va_list_tag-typedef.c

@@ -0,0 +1,14 @@
+// This test checks the patch for the compilation error / crash described in D18557.
+
+// Test as a C source
+// RUN: %clang_cc1 -emit-pch -x c-header -o %t %S/Inputs/__va_list_tag-typedef.h
+// RUN: %clang_cc1 -fsyntax-only -include-pch %t %s
+
+// Test as a C++ source
+// RUN: %clang_cc1 -emit-pch -x c++-header -o %t %S/Inputs/__va_list_tag-typedef.h
+// RUN: %clang_cc1 -x c++ -fsyntax-only -include-pch %t %s
+
+// expected-no-diagnostics
+
+typedef __builtin_va_list va_list_2;
+void test(const char* format, ...) { va_list args; va_start( args, format ); }