[NFC][flang] Inclusive language: remove instances of master

[quinnp]

Summary

[NFC] As part of using inclusive language within the llvm project, this patch:

• replaces master with main in C++style.md to match the renaming of the master

branch,

• removes master from FortranIR.md where it is superfluous,
• renames a logical variable in pre-fir-tree04.f90 containing master.

[schweitz]

flang/test/Lower/pre-fir-tree04.f90

13

Can you leave a LOGICAL variable declaration and just give the variable a new name instead? Could be a property like is_square or something.

[quinnp]

Address review comments.

[quinnp]

This comment was removed by quinnp.

flang/test/Lower/pre-fir-tree04.f90

13

Sure @schweitz, thanks for the feedback! I renamed the variable to is_square.

[quinnp]

@schweitz could you take a look at the new changes when you have time? Thanks!

flang/docs/C++style.md

 ```
 
 This document captures the style guide rules that are followed in the Flang codebase.
 
 ## In brief:
 * Use *clang-format*
 from llvm 7
 on all C++ source and header files before
-every merge to master.  All code layout should be determined
+every merge to main.  All code layout should be determined
 by means of clang-format.
 * Where a clear precedent exists in the project, follow it.
 * Otherwise, where [LLVM's C++ style guide](https://llvm.org/docs/CodingStandards.html#style-issues)
 is clear on usage, follow it.
 * Otherwise, where a good public C++ style guide is relevant and clear,
   follow it.  [Google's](https://google.github.io/styleguide/cppguide.html)
   is pretty good and comes with lots of justifications for its rules.
 * Reasonable exceptions to these guidelines can be made.

flang/docs/FortranIR.md

 * Pro: Simplification of the CFG can be done without worrying about SCOPE markers
 * Pro: Allows a precise superimposing of all Fortran constructs with scoping considerations over an otherwise ad hoc CFG.
 * Con: Adds "an extra layer" to FIR as compared to SIL. However, that can be mitigated/made inconsequential by a pass that flattens the Region tree and inserts the backstage SCOPE marker statements.
 
 #### Structure
 
 _Program_: A program instance is the top-level object that contains the representation of all the code being compiled, the compilation unit. It contains a list of procedures and a reference to the global symbol table.
 
-_Procedure_: This is a named Fortran procedure (subroutine or function). It contains a (hierarchical) list of regions. It also owns the master list of all basic blocks for the procedure.
+_Procedure_: This is a named Fortran procedure (subroutine or function). It contains a (hierarchical) list of regions. It also owns a list of all basic blocks for the procedure.
 
 _Region_: A region is owned by a procedure or by another region. A region owns a reference to a scope in the symbol table tree. The list of delineated basic blocks can also be requested from a region.
 
 _Basic block_: A basic block is owned by a procedure. A basic block owns a list of statements. The last statement in the list must be a terminator, and no other statement in the list can be a terminator. A basic block owns a list of its predecessors, which are also basic blocks. (Precisely, it is this level of FIR that is the CFG.)
 
 _Statement_: An executable Fortran construct that owns/refers to expressions, symbols, scopes, etc. produced by the front-end.
 
 _Terminator_: A statement that orchestrates control-flow. Terminator statements may reference other basic blocks and can be accessed by their parent basic block to discover successor blocks, if any.

flang/test/Lower/pre-fir-tree04.f90

 ! RUN: %flang_fc1 -fsyntax-only -fdebug-pre-fir-tree %s | FileCheck %s
 
 ! Test Pre-FIR Tree captures all the coarray related statements
 
 ! CHECK: Subroutine test_coarray
 Subroutine test_coarray
   use iso_fortran_env, only: team_type, event_type, lock_type
   type(team_type) :: t
   type(event_type) :: done
   type(lock_type) :: alock
   real :: y[10,*]
   integer :: counter[*]
-  logical :: is_master
+  logical :: is_square
   ! CHECK: <<ChangeTeamConstruct>>
   change team(t, x[5,*] => y)
     ! CHECK: AssignmentStmt
     x = x[4, 1]
   end team
   ! CHECK: <<End ChangeTeamConstruct>>
   ! CHECK: FormTeamStmt
   form team(1, t)