[MSP430] Default to unsigned char (#115964)

This matches the ABI document at https://www.ti.com/lit/pdf/slaa534 as well as the GCC implementation.

Partially fixes #115957

Author: arichardson

Diff for: clang/lib/Driver/ToolChains/Clang.cpp

@@ -1341,6 +1341,7 @@ static bool isSignedCharDefault(const llvm::Triple &Triple) {
     return false;
 
   case llvm::Triple::hexagon:
+  case llvm::Triple::msp430:
   case llvm::Triple::ppcle:
   case llvm::Triple::ppc64le:
   case llvm::Triple::riscv32:

Diff for: clang/test/Driver/msp430-char.c

@@ -0,0 +1,4 @@
+/// Check that char is unsigned by default.
+// RUN: %clang -### %s --target=msp430 -c 2>&1 | FileCheck %s
+// CHECK: "-cc1" "-triple" "msp430"
+// CHECK-SAME: "-fno-signed-char"