build: Add support for a SHOW_DIAGNOSTICS build variable.
build: Add support for a SHOW_DIAGNOSTICS build variable.

If enabled, this will attempt to use the CC_LOG_DIAGNOSTICS feature I dropped
into Clang to print a log of all the diagnostics generated during an individual
build (from the top-level). Not sure if this will actually be useful, but for
now it is handy for testing the option.

git-svn-id: a4a6f32337ebd29ad4763b423022f00f68d1c7b7@129312 91177308-0d34-0410-b5e6-96231b3b80d8

lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx