[llvm-objdump][MachO] Update check in flaky test that depends on directory path (#150674)

Earlier, function-starts.test failed when directory path contained
'_main', because objdump prints out the entire directory path, and there
is an implicit-check-not for '_main'.
There's three ways in which [function starts are printed by
llvm-objdump](https://llvm.org/docs/CommandGuide/llvm-objdump.html#cmdoption-llvm-objdump-function-starts): 

With the "addr" mode, there will be no "_main"\
With "names", it'll be printed as "_main\n"\
With "both", it'll be printed as "<addr> _main\n". 

The existing check for function-starts=addrs checks that the pattern for
"names" or "both" does not get printed. This MR changes the RUN lines to
take stdin input, so that no directory path is printed, and stray _main
strings in the directory path will not make the test fail.

Author: akshayrdeodhar

llvm/test/tools/llvm-objdump/MachO/function-starts.test

@@ -1,30 +1,34 @@
 ## This test verifies that llvm-objdump correctly prints function starts data.
 
-RUN: llvm-objdump --macho --function-starts %p/Inputs/hello.exe.macho-i386 | FileCheck %s --check-prefix=32-BIT --implicit-check-not=_main
-RUN: llvm-objdump --macho --function-starts=addrs %p/Inputs/hello.exe.macho-i386 | FileCheck %s --check-prefix=32-BIT --implicit-check-not=_main
+## Note: we read input from stdin instead of from a file, because llvm-objdump prints
+## the filename in its output. If the filename contains the string "_main", then the
+## check will incorrectly get triggered. 
+
+RUN: llvm-objdump --macho --function-starts - < %p/Inputs/hello.exe.macho-i386 | FileCheck %s --check-prefix=32-BIT --implicit-check-not=_main
+RUN: llvm-objdump --macho --function-starts=addrs - < %p/Inputs/hello.exe.macho-i386 | FileCheck %s --check-prefix=32-BIT --implicit-check-not=_main
 32-BIT: 00001f40
 
-RUN: llvm-objdump --macho --function-starts=names %p/Inputs/hello.exe.macho-i386 | FileCheck %s --check-prefix=32-BIT-NAMES
+RUN: llvm-objdump --macho --function-starts=names - < %p/Inputs/hello.exe.macho-i386 | FileCheck %s --check-prefix=32-BIT-NAMES
 32-BIT-NAMES: {{^}}_main
 
-RUN: llvm-objdump --macho --function-starts=both %p/Inputs/hello.exe.macho-i386 | FileCheck %s --check-prefix=32-BIT-BOTH
+RUN: llvm-objdump --macho --function-starts=both - < %p/Inputs/hello.exe.macho-i386 | FileCheck %s --check-prefix=32-BIT-BOTH
 32-BIT-BOTH: 00001f40 _main
 
-RUN: llvm-objdump --macho --function-starts %p/Inputs/hello.exe.macho-x86_64 | FileCheck %s --check-prefix=64-BIT --implicit-check-not=_main
-RUN: llvm-objdump --macho --function-starts=addrs %p/Inputs/hello.exe.macho-x86_64 | FileCheck %s --check-prefix=64-BIT --implicit-check-not=_main
+RUN: llvm-objdump --macho --function-starts - < %p/Inputs/hello.exe.macho-x86_64 | FileCheck %s --check-prefix=64-BIT --implicit-check-not=_main
+RUN: llvm-objdump --macho --function-starts=addrs - < %p/Inputs/hello.exe.macho-x86_64 | FileCheck %s --check-prefix=64-BIT --implicit-check-not=_main
 64-BIT: 0000000100000f30
 
-RUN: llvm-objdump --macho --function-starts=names %p/Inputs/hello.exe.macho-x86_64 | FileCheck %s --check-prefix=64-BIT-NAMES
+RUN: llvm-objdump --macho --function-starts=names - < %p/Inputs/hello.exe.macho-x86_64 | FileCheck %s --check-prefix=64-BIT-NAMES
 64-BIT-NAMES: {{^}}_main
 
-RUN: llvm-objdump --macho --function-starts=both %p/Inputs/hello.exe.macho-x86_64 | FileCheck %s --check-prefix=64-BIT-BOTH
+RUN: llvm-objdump --macho --function-starts=both - < %p/Inputs/hello.exe.macho-x86_64 | FileCheck %s --check-prefix=64-BIT-BOTH
 64-BIT-BOTH: 0000000100000f30 _main
 
-RUN: llvm-strip %p/Inputs/hello.exe.macho-x86_64 -o %t.stripped
-RUN: llvm-objdump --macho --function-starts=both %t.stripped | FileCheck %s --check-prefix=BOTH-STRIPPED
+RUN: llvm-strip  %p/Inputs/hello.exe.macho-x86_64 -o %t.stripped
+RUN: llvm-objdump --macho --function-starts=both - < %t.stripped | FileCheck %s --check-prefix=BOTH-STRIPPED
 BOTH-STRIPPED: 0000000100000f30 ?
 
 RUN: llvm-strip %p/Inputs/hello.exe.macho-x86_64 -o %t.stripped
-RUN: llvm-objdump --macho --function-starts=names %t.stripped | FileCheck %s --check-prefix=NAMES-STRIPPED
-NAMES-STRIPPED: function-starts.test.tmp.stripped:
+RUN: llvm-objdump --macho --function-starts=names - < %t.stripped | FileCheck %s --check-prefix=NAMES-STRIPPED
+NAMES-STRIPPED: -:
 NAMES-STRIPPED-EMPTY: