Prefer __vector over vector keyword for altivec

`vector' uses the keyword-and-predefine mode from gcc, while __vector is
reliably supported.

As a side effect, it also makes the code consistent in its usage of __vector.

Differential Revision: https://reviews.llvm.org/D74129

clang/lib/Lex/Lexer.cpp

@@ -2552,8 +2552,8 @@ bool Lexer::SkipBlockComment(Token &Result, const char *CurPtr,
         '/', '/', '/', '/',  '/', '/', '/', '/',
         '/', '/', '/', '/',  '/', '/', '/', '/'
       };
-      while (CurPtr+16 <= BufferEnd &&
-             !vec_any_eq(*(const vector unsigned char*)CurPtr, Slashes))
+      while (CurPtr + 16 <= BufferEnd &&
+             !vec_any_eq(*(const __vector unsigned char *)CurPtr, Slashes))
         CurPtr += 16;
 #else
       // Scan for '/' quickly.  Many block comments are very large.