MFC r259053:

Pull in r196590 from upstream clang trunk (by rdivacky):

  Move the body of GCCInstallationDetector ctor into an init() function
  and call it from its only user. The linux toolchain. This saves quite
  a lot of directory searching on other platforms.

See http://docs.freebsd.org/cgi/mid.cgi?51E6FAF5.3080802 for the
original discussion.  With this fix, the search for gcc installations is
completely eliminated on FreeBSD.

Reported by:	Kurt Lidl <lidl@pix.net>