Header guard canonicalization, clang part.

Modifications made by clang-tidy with minor tweaks.

llvm-svn: 215557
diff --git a/clang/include/clang-c/Platform.h b/clang/include/clang-c/Platform.h
index 0f866c6..e2a4dcc 100644
--- a/clang/include/clang-c/Platform.h
+++ b/clang/include/clang-c/Platform.h
@@ -11,8 +11,8 @@
 |*                                                                            *|
 \*===----------------------------------------------------------------------===*/
 
-#ifndef CLANG_C_PLATFORM_H
-#define CLANG_C_PLATFORM_H
+#ifndef LLVM_CLANG_C_PLATFORM_H
+#define LLVM_CLANG_C_PLATFORM_H
 
 #ifdef __cplusplus
 extern "C" {