Add _Complex type support to C parser

Add _Complex type support to C parser

This changes the C parser to add support for complex types in casts.

gdb/ChangeLog
2020-04-01  Tom Tromey  <tom@tromey.com>

	* c-exp.y (FLOAT_KEYWORD, COMPLEX): New tokens.
	(scalar_type): New rule, from typebase.
	(typebase): Use scalar_type.  Recognize complex types.
	(field_name): Handle FLOAT_KEYWORD.
	(ident_tokens): Add _Complex and __complex__.

gdb/testsuite/ChangeLog
2020-04-01  Tom Tromey  <tom@tromey.com>

	* gdb.base/complex-parts.exp: Add type tests.