[unittest][Support] Fix bad negation of signed integer in LEB128Test.SLEB128Size (#72700)

I came across an undefined behavior when running Support unit tests with
UBSAN on Windows.
```bash
[ RUN      ] LEB128Test.SLEB128Size
C:\llvm\unittests\Support\LEB128Test.cpp:382:3: runtime error: negation of -9223372036854775808 cannot be represented in type 'long long'; cast to an unsigned type to negate this value to itself
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior C:\llvm\unittests\Support\LEB128Test.cpp:382:3 in 
```

UBSAN did not report the same error on macOS, but the negation still
seemed invalid (`0x8000000000000000LL == -0x8000000000000000LL`
evaluated to `true`).
I can confirm that `-0x7fffffffffffffffLL - 1` results in the expected
value on both platforms.

Author: wdunicornpro

llvm/unittests/Support/LEB128Test.cpp

@@ -379,7 +379,7 @@ TEST(LEB128Test, SLEB128Size) {
   EXPECT_EQ(10u, getSLEB128Size(-0x4100000000000000LL));
 
   EXPECT_EQ(10u, getSLEB128Size(-0x7fffffffffffffffLL));
-  EXPECT_EQ(10u, getSLEB128Size(-0x8000000000000000LL));
+  EXPECT_EQ(10u, getSLEB128Size(-0x7fffffffffffffffLL - 1));
   EXPECT_EQ(10u, getSLEB128Size(INT64_MIN));
 }