[analyzer] Enable analysis of WebKit "unified sources".

Normally the analyzer begins path-sensitive analysis from functions within
the main file, even though the path is allowed to go through any functions
within the translation unit.

When a recent version of WebKit is compiled, the "unified sources" technique
is used, that assumes #including multiple code files into a single main file.
Such file would have no functions defined in it, so the analyzer wouldn't be
able to find any entry points for path-sensitive analysis.

This patch pattern-matches unified file names that are similar to those
used by WebKit and allows the analyzer to find entry points in the included
code files. A more aggressive/generic approach is being planned as well.

Differential Revision: https://reviews.llvm.org/D45839

llvm-svn: 330876