-Werror is generally suppressed in Bison-generated C/C++ code (as in all other
generated code) to silence warnings from the Bison skeleton code.  And the Clang
plugins suppress warnings in generated WORKDIR code based on the presumed source
location (i.e., taking #line directives into account).  So introduce a new
PLUGIN_WARNINGS_AS_ERRORS mode where warnings from Clang plugins are reported as
errors even if -Werror is suppressed.  That way, any warnings in the Bison
skeleton code still do not lead to compilation errors, while (at least plugin-
emitted) warnings in the genuine source code do.

Unfortunately this cannot also be enabled for Flex source code, as at least
Flex 2.5.39 generates poor code that does not properly prefix all skeleton code
with appropriate #line directives, so that some skeleton code would be mistaken
for genunie source code, and compilation would fail due to errors.

Also, %glr-parser Bison input appears to generate no #line directives at all (at
least with Bison 3.0.4), so all of connectivity/source/parse/sqlbison.y is
considered generated code and plugin warnings are still suppressed throughout.

Change-Id: Id746e81cbfa5f77628b0a34c7b82780948e7db08