Fontify CPP expressions correctly when starting in the middle of such a

Fontify CPP expressions correctly when starting in the middle of such a
construct.  Mainly for when jit-lock etc. starts a chunk here.

cc-fonts.el (c-font-lock-context): new buffer local variable.
(c-make-font-lock-search-form): new function, extracted from
c-make-font-lock-search-function.
(c-make-font-lock-search-function): Use the above function.
(c-make-font-lock-context-search-function): New function.
(c-cpp-matchers): Enhance the preprocessor expression case with the above
function
(c-font-lock-complex-decl-prepare):  Test for being in a CPP form which
takes an expression.

cc-langs.el (c-cpp-expr-intro-re): New lang-variable.