[vim/vim] Reject non-string-literal arguments to STRLEN_LITERAL (PR #21122)

**Problem:**  STRLEN_LITERAL silently expands to an expression that doesn't
	  determine a string's length if it's given a non-string-literal
	  token.
**Solution:** Modify STRLEN_LITERAL so that a non-string-literal argument
	  produces a compile error.

_Regarding character arrays:_

A compilation error will now be generated for the following:
	`char s[64] = "12345";`
	`int l = STRLEN_LITERAL(s);  // previously l was assigned 63 (not 5).`
Had `s` been defined as:
	`char s[] = "12345";`
the original definition of STRLEN_LITERAL would have correctly
determined the string length to be 5; however, the macro cannot
distinguish a padded array like the first from an exactly-sized one like
the second, so both are rejected.

_Regarding PATHSEPSTR:_

On MS-Windows, PATHSEPSTR does not expand to a string-literal token, so
STRLEN_LITERAL(PATHSEPSTR) fails to compile. Each
STRLEN_LITERAL(PATHSEPSTR) is replaced with sizeof(PATHSEP).
sizeof(PATHSEP) is now always 1; previously it was 1 (sizeof(char_u)) on
MS-Windows but sizeof(int) elsewhere. (PATHSEP is a single character;
PATHSEPSTR is a null-terminated string with a single content byte
followed by NUL.)

Supported by AI (Claude Sonnet 5).

You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/21122

-- Commit Summary --

  * Reject non-string-literal arguments to STRLEN_LITERAL
  * Fix MS-Windows build

-- File Changes --

    M src/ascii.h (2)
    M src/cmdexpand.c (2)
    M src/fileio.c (4)
    M src/filepath.c (4)
    M src/findfile.c (2)
    M src/help.c (2)
    M src/os_unix.c (2)
    M src/scriptfile.c (2)
    M src/structs.h (8)

-- Patch Links --

https://github.com/vim/vim/pull/21122.patch
https://github.com/vim/vim/pull/21122.diff

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