[clang] [clang][ASTMatcher] Add `matchesString` for `StringLiteral` which matches literals on given `RegExp` (PR #102152)

[Aaron Ballman via cfe-commits]

================
@@ -2503,6 +2503,25 @@ TEST_P(ASTMatchersTest, IsDelegatingConstructor) {
       cxxConstructorDecl(isDelegatingConstructor(), parameterCountIs(1))));
 }
 
+TEST_P(ASTMatchersTest, MatchesString) {
+  StatementMatcher Literal = stringLiteral(matchesString("foo.*"));
+  EXPECT_TRUE(matches("const char* a = \"foo\";", Literal));
+  EXPECT_TRUE(matches("const char* b = \"foobar\";", Literal));
+  EXPECT_TRUE(matches("const char* b = \"fo\"\"obar\";", Literal));
+  EXPECT_TRUE(notMatches("const char* c = \"bar\";", Literal));
+  // test embedded nulls
+  StatementMatcher Literal2 = stringLiteral(matchesString("bar"));
+  EXPECT_TRUE(matches("const char* b = \"foo\\0bar\";", Literal2));
+  EXPECT_TRUE(notMatches("const char* b = \"foo\\0b\\0ar\";", Literal2));
+  // test prefix
+  if (!GetParam().isCXX20OrLater()) {
+    return;
+  }
+  EXPECT_TRUE(matches("const wchar_t* a = L\"foo\";", Literal));
+  EXPECT_TRUE(matches("const char16_t* a = u\"foo\";", Literal));
+  EXPECT_TRUE(matches("const char32_t* a = U\"foo\";", Literal));
----------------
AaronBallman wrote:

I think this code needs to use `matchesConditionally()` instead to ensure it gets run in the correct language mode. e.g.,
```
TEST(IsExpandedFromMacro, MatchesFromCommandLine) {
  StringRef input = R"cc(
    void Test() { FOUR_PLUS_FOUR; }
  )cc";
  EXPECT_TRUE(matchesConditionally(
      input, binaryOperator(isExpandedFromMacro("FOUR_PLUS_FOUR")), true,
      {"-std=c++11", "-DFOUR_PLUS_FOUR=4+4"}));
}
```
but that said, these all exist in C++11 and later (though `L` was C++98), and C11 and later (though `L` was C99), so we shouldn't need this check at all, right?

https://github.com/llvm/llvm-project/pull/102152