Fix compatibility with z3-4.8.1

With z3-4.8.1:
../tools/clang/lib/StaticAnalyzer/Core/Z3ConstraintManager.cpp:49:40: error:
'Z3_get_error_msg_ex' was not declared in this scope
../tools/clang/lib/StaticAnalyzer/Core/Z3ConstraintManager.cpp:49:40: note:
suggested alternative: 'Z3_get_error_msg'

Formerly used Z3_get_error_msg_ex() as one could find in z3-4.7.1 states:
	"Retained function name for backwards compatibility within v4.1"
And it is implemented only as a forwarding call:
	return Z3_get_error_msg(c, err);

Differential Revision: https://reviews.llvm.org/D54391

llvm-svn: 346635