Restrict vector component access (using "." and "[]") to variables.

Chris suggested this, since it simplifies the code generator.
If this features is needed (and we don't think it is), we can revisit.

The following test case now produces an error.
[dylan:~/llvm/tools/clang] admin% cat t.c

typedef __attribute__(( ocu_vector_type(4) )) float float4;

static void test() {
    float4 vec4;

    vec4.rg.g;
    vec4.rg[1];
}
[dylan:~/llvm/tools/clang] admin% ../../Debug/bin/clang t.c
t.c:8:12: error: vector component access limited to variables
    vec4.rg.g;
           ^~
t.c:9:12: error: vector component access limited to variables
    vec4.rg[1];
           ^~~
2 diagnostics generated.

llvm-svn: 40795