Delete unused function in ClangExpressionParser

[git 65dafa83] introduced the GetBuiltinIncludePath function copied from cfe/lib/Driver/CC1Options.cpp

This function is no longer used in lldb's expression parser and I believe it is safe to remove it.


Author: Luke Drummond <luke.drummond@codeplay.com>
Differential Revision: http://reviews.llvm.org/D17266

llvm-svn: 261328