[libcxx] [test] Fix the gets-removed.verify.cpp test with Clang 21 (llvm#169235)

This fixes test errors like this, at least for a mingw target, if
building with Clang 21 instead of Clang 20, as in the CI environment:

    # .---command stderr------------
    # | error: 'expected-error' diagnostics seen but not expected:
    # | File C:\a\llvm-mingw\llvm-mingw\llvm-project\libcxx\test\std\input.output\file.streams\c.files\gets-removed.verify.cpp Line 16: cannot initialize a parameter of type 'char *' with an lvalue of type 'const char *'
    # | 1 error generated.
    # `-----------------------------
    # error: command failed with exit status: 1

This extra, unexpected diagnostic appears in Clang 21, since commit
9eef4d1 ("Remove delayed typo
expressions"). Before this, we got the expected diagnostic `error: no
member named 'gets' in namespace 'std'`, with the typo correction hint
`did you mean 'puts'?`. After this change, we get the typo correction
hint `did you mean simply 'gets'?` instead. And with the typo correction
finding `::gets`, it goes on to produce a second diagnostic about
mismatched parameter for that function.

Avoid these unexpected diagnostics by passing the right type of
parameter to the gets function.

Author: mstorsjo

libcxx/test/std/input.output/file.streams/c.files/gets-removed.verify.cpp

@@ -12,6 +12,6 @@
 
 #include <cstdio>
 
-void f(char const* str) {
+void f(char* str) {
   (void)std::gets(str); // expected-error {{no member named 'gets' in namespace 'std'}}
 }