[clang-tidy] Fix bugprone-bad-signal-to-kill-thread crash when SIGTERM is not a literal.

If `SIGTERM` is not a literal (e.g. `#define SIGTERM ((unsigned)15)`) bugprone-bad-signal-to-kill-thread check crashes.
Stack dump:
```
 #0 0x000000000217d15a llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/llvm-project/build/bin/clang-tidy+0x217d15a)
 #1 0x000000000217b17c llvm::sys::RunSignalHandlers() (/llvm-project/build/bin/clang-tidy+0x217b17c)
 #2 0x000000000217b2e3 SignalHandler(int) (/llvm-project/build/bin/clang-tidy+0x217b2e3)
 #3 0x00007f6a7efb1390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
 #4 0x000000000212ac9b llvm::StringRef::getAsInteger(unsigned int, llvm::APInt&) const (/llvm-project/build/bin/clang-tidy+0x212ac9b)
 #5 0x0000000000593501 clang::tidy::bugprone::BadSignalToKillThreadCheck::check(clang::ast_matchers::MatchFinder::MatchResult const&) (/llvm-project/build/bin/clang-tidy+0x593501)
```

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D85398

Author: ArcsinX

clang-tools-extra/clang-tidy/bugprone/BadSignalToKillThreadCheck.cpp

@@ -39,6 +39,8 @@ void BadSignalToKillThreadCheck::check(const MatchFinder::MatchResult &Result) {
       return llvm::None;
     const MacroInfo *MI = PP->getMacroInfo(It->first);
     const Token &T = MI->tokens().back();
+    if (!T.isLiteral())
+      return llvm::None;
     StringRef ValueStr = StringRef(T.getLiteralData(), T.getLength());
 
     llvm::APInt IntValue;

clang-tools-extra/test/clang-tidy/checkers/bugprone-bad-signal-to-kill-thread-sigterm-not-a-literal.cpp

@@ -0,0 +1,10 @@
+// RUN: clang-tidy %s --checks=-*,bugprone-bad-signal-to-kill-thread -- | count 0
+
+#define SIGTERM ((unsigned)15) // no-crash
+using pthread_t = int;
+int pthread_kill(pthread_t thread, int sig);
+
+int func() {
+  pthread_t thread;
+  return pthread_kill(thread, 0);
+}