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

[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.

Reviewed By: ZarkoCA

Differential Revision: https://reviews.llvm.org/D113923

flang/docs/C++style.md

@@ -19,7 +19,7 @@ This document captures the style guide rules that are followed in the Flang code
 * 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)

flang/docs/FortranIR.md

@@ -145,7 +145,7 @@ Once the basic blocks are constructed, CFG edges defined, and the CFG is simplif
 
 _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.
 

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

@@ -10,7 +10,7 @@ Subroutine test_coarray
   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