Add validation nodes to ninja

A common problem in the Android build is inserting rules that perform
some sort of error checking that doesn't produce any artifacts needed
by the build, for example static analysis tools.  There are a few
patterns currently used, both of which have downsides.

The first is to have a rule that depends on all of the static analysis
results.  This ensures they run, but requires running static analysis
over everything, and not just the active parts of the build graph.

The second is to insert the static analysis rule into the build graph
between the artifact producing rule and anything that depends on it,
often copying the artifact as the output of the static analysis rule.
This increases the critical path of the build, often reducing
parallelism.  In the case of copying the artifact, it also wastes
disk space.

This patch adds "validation nodes" to edges in Ninja.  A build
statement can specify validation nodes using "|@" in the edge
inputs.  The validation nodes are not used as an input to the edge
(the edge can run before the validation node is ready), but are
added to the initial nodes of the build graph whenever the edge
is part of the build graph.  The edge that outputs the validation
node can depend on the output of the edge that is being validated
if desired.

Test: ninja_test
Change-Id: Ife27086c50c1b257a26509373199664680b2b247