[docs] Fix vector literal examples that don't compile

[ahatanak]

Summary

Guard the AltiVec vector variables with #ifdef __VEC__ and remove the last line, which doesn't compile.

rdar://problem/34411929

[ahatanak]

The parenthesis initialization does not work as documented for GCC or OpenCL vectors, so that has to be fixed too.

clang/docs/LanguageExtensions.rst

 .. code-block:: c++
 
   typedef int v4si __attribute__((__vector_size__(16)));
   typedef float float4 __attribute__((ext_vector_type(4)));
   typedef float float2 __attribute__((ext_vector_type(2)));
 
   v4si vsi = (v4si){1, 2, 3, 4};
   float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
+  #ifdef __VEC__
   vector int vi1 = (vector int)(1);    // vi1 will be (1, 1, 1, 1).
   vector int vi2 = (vector int){1};    // vi2 will be (1, 0, 0, 0).
   vector int vi3 = (vector int)(1, 2); // error
   vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
   vector int vi5 = (vector int)(1, 2, 3, 4);
-  float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));
+  #endif
 
 Vector Operations
 -----------------
 
 The table below shows the support for each operation by vector extension.  A
 dash indicates that an operation is not accepted according to a corresponding
 specification.