Enable -Werror-implicit-function-declaration by default (NFC)

[mehdi_amini]

Summary

This is useful in C source files where it is easy for a typo to be
silently assumed by the compiler to be an implicit declaration.

[stellaraccident]

Thank you - TIL! This has caught me but I didn't know there was a warning flag for it.

mlir/CMakeLists.txt

 # MLIR project.
 set(MLIR_MAIN_SRC_DIR     ${CMAKE_CURRENT_SOURCE_DIR}  )
 set(MLIR_MAIN_INCLUDE_DIR ${MLIR_MAIN_SRC_DIR}/include )
 
 set(MLIR_SOURCE_DIR  ${CMAKE_CURRENT_SOURCE_DIR})
 set(MLIR_BINARY_DIR  ${CMAKE_CURRENT_BINARY_DIR})
 set(MLIR_INCLUDE_DIR ${CMAKE_CURRENT_BINARY_DIR}/include)
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 include(AddMLIR)
 
+# Forbid implicit function declaration: this may lead to subtle bugs and we
+# don't have a reason to support this.
+add_flag_if_supported("-Werror-implicit-function-declaration" WERROR_IMPLICIT_FUNCTION_DECLARATION)
+
 # Installing the headers and docs needs to depend on generating any public
 # tablegen'd targets.
 # mlir-generic-headers are dialect-independent.
 add_custom_target(mlir-generic-headers)
 set_target_properties(mlir-generic-headers PROPERTIES FOLDER "Misc")
 # mlir-headers may be dialect-dependent.
 add_custom_target(mlir-headers)
 set_target_properties(mlir-headers PROPERTIES FOLDER "Misc")