CLJS-1759: Errors writing transit analysis cache if parallel build

Adds a coarse-grained mutex around the calls to write out transit
analysis cache data. A result of this is that sporadic failures that
only occur when concurrently writing out analysis cache info when
compiling largeish multi-file projects with :parallel-build true is
avoided.

(Perhaps there is some thread-safety defect in the transit-clj stack
yet to be discovered / fixed, and this patch acts as a suitable
workaround until a proper solution is found.)

src/main/clojure/cljs/analyzer.cljc

@@ -3110,6 +3110,9 @@
             true
             (util/changed? src cache)))))))
 
+#?(:clj
+   (def transit-write-mutex (Object.)))
+
 #?(:clj
    (defn write-analysis-cache
      ([ns cache-file]
@@ -3124,10 +3127,11 @@
                      (str ";; Analyzed by ClojureScript " (util/clojurescript-version) "\n"))
                        (pr-str analysis)))
           "json" (when-let [{:keys [writer write]} @transit]
-                   (write
-                     (writer (FileOutputStream. cache-file) :json
-                       transit-write-opts)
-                     analysis))))
+                   (locking transit-write-mutex
+                     (write
+                       (writer (FileOutputStream. cache-file) :json
+                         transit-write-opts)
+                       analysis)))))
       (when src
         (.setLastModified ^File cache-file (util/last-modified src))))))