[clang-format][NFC] Use `empty()` instead of comparing size() to 0 or 1

clang/lib/Format/FormatTokenLexer.cpp

diff --git a/clang/lib/Format/FormatTokenLexer.cpp b/clang/lib/Format/FormatTokenLexer.cpp
index 05dee26..40b62b2 100644
--- a/clang/lib/Format/FormatTokenLexer.cpp
+++ b/clang/lib/Format/FormatTokenLexer.cpp
@@ -488,7 +488,7 @@
 
 // In C# transform identifier foreach into kw_foreach
 bool FormatTokenLexer::tryTransformCSharpForEach() {
-  if (Tokens.size() < 1)
+  if (Tokens.empty())
     return false;
   auto &Identifier = *(Tokens.end() - 1);
   if (Identifier->isNot(tok::identifier))
@@ -948,7 +948,7 @@
   // 4. The first non-digit character is 'x', and the next is a hex digit.
   // Note that in the case 3 and 4, if the next character does not exists in
   // this token, the token is an identifier.
-  if (Text.size() < 1 || Text[0] == '+' || Text[0] == '-')
+  if (Text.empty() || Text[0] == '+' || Text[0] == '-')
     return;
   const auto NonDigitPos = Text.find_if([](char C) { return !isdigit(C); });
   // All the characters are digits

clang/lib/Format/MacroExpander.cpp

diff --git a/clang/lib/Format/MacroExpander.cpp b/clang/lib/Format/MacroExpander.cpp
index 37c25e6..85a53c9 100644
--- a/clang/lib/Format/MacroExpander.cpp
+++ b/clang/lib/Format/MacroExpander.cpp
@@ -226,7 +226,7 @@
     New->MacroCtx = MacroExpansion(MR_Hidden);
     pushToken(New);
   }
-  assert(Result.size() >= 1 && Result.back()->is(tok::eof));
+  assert(!Result.empty() && Result.back()->is(tok::eof));
   if (Result.size() > 1) {
     ++Result[0]->MacroCtx->StartOfExpansion;
     ++Result[Result.size() - 2]->MacroCtx->EndOfExpansion;

clang/lib/Format/UnwrappedLineParser.cpp

diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
index c961ce7..7e8634a 100644
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -4601,7 +4601,7 @@
   } else {
     // At the top level we only get here when no unexpansion is going on, or
     // when conditional formatting led to unfinished macro reconstructions.
-    assert(!Reconstruct || (CurrentLines != &Lines) || PPStack.size() > 0);
+    assert(!Reconstruct || (CurrentLines != &Lines) || !PPStack.empty());
     CurrentLines->push_back(std::move(*Line));
   }
   Line->Tokens.clear();