refactor/cleanup ClangExpressionParser::Parse

This patches does the following:

+ fix return type: ClangExpressionParser::Parse returns unsigned, but was actually returning a signed value, num_errors.
+ use helper clang::TextDiagnosticBuffer::getNumErrors() instead of counting the errors ourself.
+ limit scoping of block-level automatic variables as much as practical.
+ remove reused multipurpose TextDiagnosticBuffer::const_iterator in favour of loop-scoped err, warn, and note variables in the diagnostic printing code.
+ refactor diagnostic printing loops to use a proper loop invariant.

Author: Luke Drummond <luke.drummond@codeplay.com>
Differential Revision: http://reviews.llvm.org/D17273

llvm-svn: 261345