[clang] [clang-tools-extra] Reland [clang][Sema, Lex, Parse] Preprocessor embed in C and C++ (PR #95802)

[Mariya Podchishchaeva via cfe-commits]

================
@@ -0,0 +1,98 @@
+// RUN: %clang_cc1 %s -fsyntax-only --embed-dir=%S/Inputs -verify=expected,cxx -Wno-c23-extensions
+// RUN: %clang_cc1 -x c -std=c23 %s -fsyntax-only --embed-dir=%S/Inputs -verify=expected,c
+#embed <media/empty>
+;
+
+void f (unsigned char x) { (void)x;}
+void g () {}
+void h (unsigned char x, int y) {(void)x; (void)y;}
+int i () {
+	return
+#embed <single_byte.txt>
+		;
+}
+
+_Static_assert(
+#embed <single_byte.txt> suffix(,)
+""
+);
+_Static_assert(
+#embed <single_byte.txt>
+, ""
+);
+_Static_assert(sizeof(
+#embed <single_byte.txt>
+) ==
+sizeof(unsigned char)
+, ""
+);
+_Static_assert(sizeof
+#embed <single_byte.txt>
+, ""
+);
+_Static_assert(sizeof(
+#embed <jk.txt> // expected-warning {{left operand of comma operator has no effect}}
+) ==
+sizeof(unsigned char)
+, ""
+);
+
+#ifdef __cplusplus
+template <int First, int Second>
+void j() {
+	static_assert(First == 'j', "");
+	static_assert(Second == 'k', "");
+}
+#endif
+
+void do_stuff() {
+	f(
+#embed <single_byte.txt>
+	);
+	g(
+#embed <media/empty>
+	);
+	h(
+#embed <jk.txt>
+	);
+	int r = i();
+	(void)r;
+#ifdef __cplusplus
+	j<
+#embed <jk.txt>
+	>(
+#embed <media/empty>
+	);
+#endif
+}
+
+// Ensure that we don't accidentally allow you to initialize an unsigned char *
+// from embedded data; the data is modeled as a string literal internally, but
+// is not actually a string literal.
+const unsigned char *ptr =
+#embed <jk.txt> // expected-warning {{left operand of comma operator has no effect}}
+; // c-error at -2 {{incompatible integer to pointer conversion initializing 'const unsigned char *' with an expression of type 'unsigned char'}} \
+     cxx-error at -2 {{cannot initialize a variable of type 'const unsigned char *' with an rvalue of type 'unsigned char'}}
+
+// However, there are some cases where this is fine and should work.
+const unsigned char *null_ptr_1 =
+#embed <media/empty> if_empty(0)
+;
+
+const unsigned char *null_ptr_2 =
+#embed <null_byte.bin>
+;
+
+const unsigned char *null_ptr_3 = {
+#embed <null_byte.bin>
+};
+
+#define FILE_NAME <null_byte.bin>
+#define LIMIT 1
+#define OFFSET 0
+#define EMPTY_SUFFIX suffix()
+
+constexpr unsigned char ch =
+#embed FILE_NAME limit(LIMIT) clang::offset(OFFSET) EMPTY_SUFFIX
+;
+static_assert(ch == 0);
----------------
Fznamznon wrote:

This is one more case, that would be solved by injecting tokens back into the stream. Right now it is quite complex to understand that embed met by `ParseCastExpression` should be expanded in a special way.

https://github.com/llvm/llvm-project/pull/95802