VStudio compiler errors and placing Function*->ExFunc map under ManagedStatic control.

This commit fixes two things.  One is a pair of VStudio compiler errors stemming from variables
which defined within the for loop statement and also within the body of the for loop.  I fixed these 
by renaming one of the two variables.  Additionally, I've made the Function*->ExFunc map in 
ExternalFunctions.cpp a ManagedStatic object, so that cleanup will be done on llvm_shutdown.  In repeated
uses of the interpreter, where the same Function* address may get used for completely differnet functions,
this was causing a crash.

llvm-svn: 40558