Two typeof() related changes...

- Changed the name of ASTContext::getTypeOfType(Expr*)->getTypeOfExpr().
- Remove FIXME for TypeOfExpr::getAsStringInternal(). This will work fine for printing the AST. It isn't ideal
for error diagnostics (since it's more natural to display the expressions type). 

One "random" (or at least delayed:-) change...

- Changed all "ext_typecheck_*" diagnostics from EXTENSION->WARNING. Reason: Since -pedantic is now
off (by default), these diagnostics were never being emitted (which is bad). With this change, clang will
emit the warning all the time. The only downside (wrt GCC compatibility) is -pedantic-errors will not turn
this diagnostics into errors (a "feature" of making tagging them with EXTENSION). When/if this becomes
an issue, we can revisit.

llvm-svn: 40676