...to check that loplugin produces warnings/errors as expected:

* Clang has a -verify switch that makes it easy to write test input .cxx files
  that list in comments all the warnings/errors that are expected, and let Clang
  check those expectations instead of generating object code.  See
  include/clang/Frontend/VerifyDiagnosticConsumer.h in the Clang source tree for
  documentation.

* Introduce a CompilerTest gbuild class that uses the existing LinkTarget class
  as much as possible.  Checking the input files is implicitly phony, as the
  compilation step doesn't generate any object files, and the link step does
  nothing because there is no gb_LinkTarget_set_targettype for CompilerTest.
  The setup at least works for Clang on Linux (will need adaptions for Clang on
  Windows; compilers other than Clang are not relevant for now given this is
  used to check compilerplugins).

* Definition of gb_CFLAGS_WERROR in solenv/gbuild/platform/com_GCC_defs.mk needs
  to be lazy ('=' vs. ':=') so that CompilerTest can override it:  The Clang
  -verify mode wants the input files to specify whether the loplugin diagnostics
  are warnings or errros, so they consistently need to be errors independent of
  --enable-werror configuration.

* A first (example) test is in compilerplugins/clang/test/salbool.cxx.  The
  corresponding gbuild CompilerTest instance is in
  solenv/CompilerTest_compilerplugins_clang.mk for now.  The reason for that odd
  split across compilerplugins/ and solenv/ is that there is no
  compilerplugins/Modules_compilerplugins.mk file, so this setup is the easiest
  hack for now (to be cleaned up).  (Another area that could be improved is that
  all test files need to be listed explicitly in the CompilerTest_*.mk file,
  instead of, say, using all .c/.cxx/.m/.mm files in a specified directory.)

* The test is run somewhat late during a top-level 'make', after loplugin has
  already been used in compilation.  But it can be run manually (e.g., 'make
  solenv') when making changes to loplugin during development.

Change-Id: I01e12fb84887d264ac03ef2484807458c2075af4