Provide both signed and unsigned int variants of bitcode-based builtins.

When creating function Symbols for functions that were defined in LLVM bitcode for the standard library, if any of the function parameters are integer types, create two ispc-side Symbols: one where the integer types are all signed and the other where they are all unsigned.  This allows us to provide, for example, both store_to_int16(reference int a[], uniform int offset, int val) as well as store_to_int16(reference unsigned int a[], uniform int offset, unsigned int val). functions.

Added some additional tests to exercise the new variants of these.

Also fixed some cases where the __{load,store}_int{8,16} builtins would read from/write to memory even if the mask was all off (which could cause crashes in some cases.)

docs/ReleaseNotes.txt

@@ -1,5 +1,11 @@
 === v1.0.3 === (not yet released)
 
+There are now both 'signed' and 'unsigned' variants of the standard library
+functions like packed_load_active() that that references to arrays of
+signed int32s and unsigned int32s respectively.  (The
+{load_from,store_to}_{int8,int16}() functions have similarly been augmented
+to have both 'signed' and 'unsigned' variants.)
+
 In initializer expressions with variable declarations, it is no longer
 legal to initialize arrays and structs with single scalar values that then
 initialize their members; they now must be initialized with initializer