swift234

Re-enable building of static libraries.

Add a proper build option SWIFT_BUILD_STATIC to support building
static libraries. Default this option to the old build setting,
ENABLE_SWIFT_STATIC_RUNTIME, which has an unfortunate name and was
never a proper build option. <rdar://problem/17648470>

This commit is only sufficient for building static libraries for
non-SDK builds. SDK build support to follow.

Note that Xcode projects built with SWIFT_BUILD_STATIC don't quite
work: Xcode refuses to build the static library if it doesn't contain
any files that Xcode itself builds, e.g., if there are no
.m/.c/.mm/.cpp files in the target.

Swift SVN r19940