[lldb][NFC] Return a reference from ClangASTContext::getASTContext and remove dead nullptr checks

ClangASTContext::getASTContext() currently returns a ptr but we have an assert there since a
while that the ASTContext is not a nullptr. This causes that we still have a lot of code
that is doing nullptr checks on the result of getASTContext() which is all unreachable code.

This patch changes the return value to a reference to make it clear this can't be a nullptr
and deletes all the nullptr checks.
diff --git a/lldb/source/Plugins/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp b/lldb/source/Plugins/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp
index 08e3737..69a4470 100644
--- a/lldb/source/Plugins/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp
+++ b/lldb/source/Plugins/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp
@@ -413,8 +413,7 @@
 
     ClangASTContext *ast_ctx =
         ClangASTContext::GetScratch(m_process->GetTarget());
-    if (ast_ctx->getASTContext() &&
-        m_dispatch_tsd_indexes_addr != LLDB_INVALID_ADDRESS) {
+    if (m_dispatch_tsd_indexes_addr != LLDB_INVALID_ADDRESS) {
       CompilerType uint16 =
           ast_ctx->GetBuiltinTypeForEncodingAndBitSize(eEncodingUint, 16);
       CompilerType dispatch_tsd_indexes_s = ast_ctx->CreateRecordType(