Fix broken builtin functions in the expression command

Summary:
Calling any non-libc builtin function in the expression command currently just causes Clang
to state that the function is not known. The reason for this is that we actually never
initialize the list of builtin functions in the Builtin::Context.

This patch just calls the initializer for the builtins in the preprocessor. Also adds some tests
for the new builtins.

It also gets rid of the extra list of builtins in the ClangExpressionParser, as we can just reuse
the existing list in the Preprocessor for the ASTContext. Having just one list of builtins around
is also closer to the standard Clang behavior.

Reviewers: #lldb, vsk

Reviewed By: vsk

Subscribers: sgraenitz, clayborg, vsk, lldb-commits

Differential Revision: https://reviews.llvm.org/D50481

llvm-svn: 340571