[Static Analyzer] Add --analyzer-target option to scan-build.

When interposing on a compiler doing cross-compilation, scan-build
does not infer the target triple needed to pass to clang for
doing static analysis.  The --analyzer-target option allows one
to manually specify the target triple used during static analysis
(and only static analysis) for such cases.

Patch by Honggyu Kim!

Reviewed in http://reviews.llvm.org/D10356.

llvm-svn: 244400