[C11/C2x] Change the behavior of the implicit function declaration warning

C89 had a questionable feature where the compiler would implicitly
declare a function that the user called but was never pr

[C11/C2x] Change the behavior of the implicit function declaration warning

C89 had a questionable feature where the compiler would implicitly
declare a function that the user called but was never previously
declared. The resulting function would be globally declared as
extern int func(); -- a function without a prototype which accepts zero
or more arguments.

C99 removed support for this questionable feature due to severe
security concerns. However, there was no deprecation period; C89 had
the feature, C99 didn't. So Clang (and GCC) both supported the
functionality as an extension in C99 and later modes.

C2x no longer supports that function signature as it now requires all
functions to have a prototype, and given the known security issues with
the feature, continuing to support it as an extension is not tenable.

This patch changes the diagnostic behavior for the
-Wimplicit-function-declaration warning group depending on the language
mode in effect. We continue to warn by default in C89 mode (due to the
feature being dangerous to use). However, because this feature will not
be supported in C2x mode, we've diagnosed it as being invalid for so
long, the security concerns with the feature, and the trivial
workaround for users (declare the function), we now default the
extension warning to an error in C99-C17 mode. This still gives users
an easy workaround if they are extensively using the extension in those
modes (they can disable the warning or use -Wno-error to downgrade the
error), but the new diagnostic makes it more clear that this feature is
not supported and should be avoided. In C2x mode, we no longer allow an
implicit function to be defined and treat the situation the same as any
other lookup failure.

Differential Revision: https://reviews.llvm.org/D122983

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Use functions with prototypes when appropriate; NFC

A significant number of our tests in C accidentally use functions
without prototypes. This patch converts the function signatures to have
a protot

Use functions with prototypes when appropriate; NFC

A significant number of our tests in C accidentally use functions
without prototypes. This patch converts the function signatures to have
a prototype for the situations where the test is not specific to K&R C
declarations. e.g.,

void func();

becomes

void func(void);

This is the twelfth batch of tests being updated (the end may be in
sight soon though).

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Update tests to use %clang_cc1 instead of 'clang-cc' or 'clang -cc1'.
- This is designed to make it obvious that %clang_cc1 is a "test variable"
which is substituted. It is '%clang_cc1' instead o

Update tests to use %clang_cc1 instead of 'clang-cc' or 'clang -cc1'.
- This is designed to make it obvious that %clang_cc1 is a "test variable"
which is substituted. It is '%clang_cc1' instead of '%clang -cc1' because it
can be useful to redefine what gets run as 'clang -cc1' (for example, to set
a default target).

llvm-svn: 91446

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Rename clang to clang-cc.

Tests and drivers updated, still need to shuffle dirs.

llvm-svn: 67602

eliminate some random .ll file outputs

llvm-svn: 63117

Update a number of CodeGen tests to not create .ll files in the test
directory.
- Removed .ll from the svn:ignore lists to try and prevent this.
- Added svn:ignore on test/Misc/Output

llvm-svn: 55

Update a number of CodeGen tests to not create .ll files in the test
directory.
- Removed .ll from the svn:ignore lists to try and prevent this.
- Added svn:ignore on test/Misc/Output

llvm-svn: 55104

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Fix typo in writable string test

llvm-svn: 44398

Adding code gen tests for writable and shared string literals.

llvm-svn: 44397