A following patch will include common/preprocessor.h in some .c file
that also includes readline.h, and that revealed a conflict -- ESC is
defined by readline.h as well (actually readline's chardefs.h) with a
completely unrelated meaning:

 #define ESC CTRL('[')

Rename our version to avoid the conflict.

gdb/ChangeLog:
2019-01-23  Pedro Alves  <palves@redhat.com>

	* common/preprocessor.h (ESC): Rename to ...
	(ESC_PARENS): ... this.
	* common/valid-expr.h (CHECK_VALID_EXPR_1, CHECK_VALID_EXPR_2)
	(CHECK_VALID_EXPR_3, CHECK_VALID_EXPR_4): Adjust.