Avoid undefined arithmetic shifting

The result of a left shift on a positive signed integer (Rune) must fit
into an unsigned integer otherwise it's undefined behaviour, as is left
shifting a negative integer by any amount. This code can only be hit if
|x >= 0xf0| and hence a left shift of 31 will always be undefined
unless the input is 0 or 1.

To avoid hitting this we can instead extend the lowest bit to be the
mask if we assume that ints are 2's complement, which we already do
elsewhere. This generates identical code in testing on Compiler
Explorer and the Odin test suite passes locally with this change.

Note that the original code would change to be defined behaviour in
C++20, however we are currently build with |-std=c++14| in the build
scripts.

src/unicode.cpp

@@ -109,7 +109,7 @@ gb_internal isize utf8_decode(u8 const *str, isize str_len, Rune *codepoint_out)
 		u8 b1, b2, b3;
 		Utf8AcceptRange accept;
 		if (x >= 0xf0) {
-			Rune mask = (cast(Rune)x << 31) >> 31;
+			Rune mask = -cast(Rune)(x & 1);
 			codepoint = (cast(Rune)s0 & (~mask)) | (GB_RUNE_INVALID & mask);
 			width = 1;
 			goto end;