Bug 45973 - Inaccurate system requirements

[jnyfah]

Summary

Update minimum Clang and GCC software requirements for building LLVM

[jnyfah]

Pulled rebase from llvm main

[xgupta]

Adding @steveire as a reviewer because he is the author of the original RFC proposes a few days back.

[steveire]

Hi, thanks for this. I'm not sure there was enough assent on the mailing list or agreement on the direction, but it might indeed make sense to put a patch together to discuss it.

Do we know that Clang builds with the compiler versions in this patch?

llvm/docs/GettingStarted.rst

 Package                                                     Version      Notes
 =========================================================== ============ ==========================================
 `CMake <http://cmake.org/>`__                               >=3.13.4     Makefile/workspace generator
-`GCC <http://gcc.gnu.org/>`_                                >=5.1.0      C/C++ compiler\ :sup:`1`
+`GCC <http://gcc.gnu.org/>`_                                >=7.4      C/C++ compiler\ :sup:`1`
 `python <http://www.python.org/>`_                          >=3.6        Automated test suite\ :sup:`2`
 `zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`3`
 `GNU Make <http://savannah.gnu.org/projects/make>`_         3.79, 3.79.1 Makefile/build processor\ :sup:`4`

 standards<CodingStandards>`. To enforce this language version, we check the most
 popular host toolchains for specific minimum versions in our build systems:
 
-* Clang 3.5
+* Clang 6.0
 * Apple Clang 6.0
-* GCC 5.1
+* GCC 7.4
 * Visual Studio 2017
 
 Anything older than these toolchains *may* work, but will require forcing the