"bool" should be a context-sensitive keyword in Altivec mode.

PR16456 reported that Clang implements a hybrid between AltiVec's
"Keyword and Predefine Method" and its "Context Sensitive Keyword
Method," where "bool" is always a keyword, but "vector" and "pixel"
are context-sensitive keywords.  This isn't permitted by the AltiVec
spec.  For consistency with gcc, this patch implements the Context
Sensitive Keyword Method for bool, and stops treating true and false
as keywords in Altivec mode.

The patch removes KEYALTIVEC as a trigger for defining these keywords
in include/clang/Basic/TokenKinds.def, and adds logic for "vector
bool" that mirrors the existing logic for "vector pixel."  The test
case is taken from the bug report.