Fix scope-based lookup when more than one function is found.

When multiple functions are found by name, lldb removes duplicate entries of
functions with the same type, so the first function in the symbol context list
is chosen, even if it isn't in scope. This patch uses the declaration context
of the execution context to select the function which is in scope.

This fixes cases like the following:

    int func();
    namespace ns {
	int func();
	void here() {
	    // Run to BP here and eval 'p func()';
	    // lldb used to find ::func(), now finds ns::func().
	}
    }

Reviewed by: clayborg
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D15312

llvm-svn: 255439