Fix CLANG_ENABLE_STATIC_ANALYZER=OFF building all analyzer source

Since https://reviews.llvm.org/D87118, the StaticAnalyzer directory is
added unconditionally. In theory this should not cause the static analyzer
sources to be built unless they are referenced by another target. However,
the clang-cpp target (defined in clang/tools/clang-shlib) uses the
CLANG_STATIC_LIBS global property to determine which libraries need to
be included. To solve this issue, this patch avoids adding libraries to
that property if EXCLUDE_FROM_ALL is set.

In case something like this comes up again: `cmake --graphviz=targets.dot`
is quite useful to see why a target is included as part of `ninja all`.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D109611